Add C++ interop inline function tests (#5406)

This shows that `inline` is ignored and the function definition is not
generated.

Demo:

```c++
// hello_world.h

inline void hello_world() {}
```

```carbon
// main.carbon

library "Main";

import Cpp library "hello_world.h";

fn Run() -> i32 {
  Cpp.hello_world();
  return 0;
}
```

```shell
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link main.o --output=demo
ld.lld: error: undefined symbol: hello_world()
>>> referenced by main.carbon:8
>>>               main.o:(main)
error: linker command failed with exit code 1 (use -v to see invocation)
```

Part of #5405.

toolchain/check/testdata/interop/cpp/no_prelude/function_decl_inline.carbon

@@ -0,0 +1,131 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/no_prelude/function_decl_inline.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/no_prelude/function_decl_inline.carbon
+
+// ============================================================================
+// inline_function_decl_with_definition
+// ============================================================================
+
+// --- inline_function_decl_with_definition.h
+
+inline void foo() {}
+
+// --- import_inline_function_decl_with_definition.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "inline_function_decl_with_definition.h";
+
+fn MyF() {
+  Cpp.foo();
+}
+
+// ============================================================================
+// inline_function_decl_without_definition
+// ============================================================================
+
+// --- inline_function_decl_without_definition.h
+
+inline void foo();
+
+// --- todo_fail_import_inline_function_decl_without_definition.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "inline_function_decl_without_definition.h";
+
+fn MyF() {
+  // TODO: Error on using an inline function without definition.
+  Cpp.foo();
+
+  // Don't error on repeated calls.
+  Cpp.foo();
+}
+
+// CHECK:STDOUT: --- import_inline_function_decl_with_definition.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %MyF.type: type = fn_type @MyF [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %MyF: %MyF.type = struct_value () [concrete]
+// CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
+// CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Cpp: <namespace> = namespace file.%Cpp.import_cpp, [concrete] {
+// CHECK:STDOUT:     .foo = @MyF.%foo.decl
+// CHECK:STDOUT:     import Cpp//...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Cpp = imports.%Cpp
+// CHECK:STDOUT:     .MyF = %MyF.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Cpp.import_cpp = import_cpp {
+// CHECK:STDOUT:     import Cpp "inline_function_decl_with_definition.h"
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %MyF.decl: %MyF.type = fn_decl @MyF [concrete = constants.%MyF] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @MyF() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %Cpp.ref: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %foo.decl: %foo.type = fn_decl @foo [concrete = constants.%foo] {} {}
+// CHECK:STDOUT:   %foo.ref: %foo.type = name_ref foo, %foo.decl [concrete = constants.%foo]
+// CHECK:STDOUT:   %foo.call: init %empty_tuple.type = call %foo.ref()
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @foo();
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- todo_fail_import_inline_function_decl_without_definition.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %MyF.type: type = fn_type @MyF [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %MyF: %MyF.type = struct_value () [concrete]
+// CHECK:STDOUT:   %foo.type: type = fn_type @foo [concrete]
+// CHECK:STDOUT:   %foo: %foo.type = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Cpp: <namespace> = namespace file.%Cpp.import_cpp, [concrete] {
+// CHECK:STDOUT:     .foo = @MyF.%foo.decl
+// CHECK:STDOUT:     import Cpp//...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Cpp = imports.%Cpp
+// CHECK:STDOUT:     .MyF = %MyF.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Cpp.import_cpp = import_cpp {
+// CHECK:STDOUT:     import Cpp "inline_function_decl_without_definition.h"
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %MyF.decl: %MyF.type = fn_decl @MyF [concrete = constants.%MyF] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @MyF() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %Cpp.ref.loc8: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %foo.decl: %foo.type = fn_decl @foo [concrete = constants.%foo] {} {}
+// CHECK:STDOUT:   %foo.ref.loc8: %foo.type = name_ref foo, %foo.decl [concrete = constants.%foo]
+// CHECK:STDOUT:   %foo.call.loc8: init %empty_tuple.type = call %foo.ref.loc8()
+// CHECK:STDOUT:   %Cpp.ref.loc11: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %foo.ref.loc11: %foo.type = name_ref foo, %foo.decl [concrete = constants.%foo]
+// CHECK:STDOUT:   %foo.call.loc11: init %empty_tuple.type = call %foo.ref.loc11()
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @foo();
+// CHECK:STDOUT:

toolchain/lower/testdata/interop/cpp/function_decl.carbon

@@ -8,6 +8,10 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/lower/testdata/interop/cpp/function_decl.carbon
 
+// ============================================================================
+// function_decl
+// ============================================================================
+
 // --- function_decl.h
 
 void foo();
@@ -22,6 +26,25 @@ fn MyF() {
   Cpp.foo();
 }
 
+// ============================================================================
+// inline_function_decl
+// ============================================================================
+
+// --- inline_function_decl.h
+
+inline void foo() {}
+
+// --- todo_import_inline_function_decl.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp library "inline_function_decl.h";
+
+fn MyF() {
+  // TODO: This should generate the definition of the inline function `foo()`.
+  Cpp.foo();
+}
+
 // CHECK:STDOUT: ; ModuleID = 'import_function_decl.carbon'
 // CHECK:STDOUT: source_filename = "import_function_decl.carbon"
 // CHECK:STDOUT:
@@ -45,3 +68,26 @@ fn MyF() {
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 7, column: 3, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 6, column: 1, scope: !4)
+// CHECK:STDOUT: ; ModuleID = 'todo_import_inline_function_decl.carbon'
+// CHECK:STDOUT: source_filename = "todo_import_inline_function_decl.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: define void @_CMyF.Main() !dbg !4 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   call void @_Z3foov(), !dbg !7
+// CHECK:STDOUT:   ret void, !dbg !8
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare void @_Z3foov()
+// CHECK:STDOUT:
+// CHECK:STDOUT: !llvm.module.flags = !{!0, !1}
+// CHECK:STDOUT: !llvm.dbg.cu = !{!2}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
+// CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
+// CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
+// CHECK:STDOUT: !3 = !DIFile(filename: "todo_import_inline_function_decl.carbon", directory: "")
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "MyF", linkageName: "_CMyF.Main", scope: null, file: !3, line: 6, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
+// CHECK:STDOUT: !6 = !{}
+// CHECK:STDOUT: !7 = !DILocation(line: 8, column: 3, scope: !4)
+// CHECK:STDOUT: !8 = !DILocation(line: 6, column: 1, scope: !4)