Properly dump SemIR for inline C++ imports (#6001)

Dumping SemIR crashed on inline C++ imports and this outputs `import Cpp
inline` instead.
Followup of #5904.

toolchain/check/testdata/interop/cpp/inline.carbon

@@ -14,12 +14,14 @@
 
 library "[[@TEST_NAME]]";
 
+//@dump-sem-ir-begin
 import Cpp inline '''
 
 // A C++ function.
 inline void func() {}
 
 ''';
+//@dump-sem-ir-end
 
 fn Run() {
   //@dump-sem-ir-begin
@@ -31,12 +33,14 @@ fn Run() {
 
 library "[[@TEST_NAME]]";
 
+//@dump-sem-ir-begin
 import Cpp inline '''c++
 
 // A C++ function.
 inline void another_func() {}
 
 ''';
+//@dump-sem-ir-end
 
 fn Run() {
   //@dump-sem-ir-begin
@@ -60,6 +64,12 @@ fn Run() {
 // CHECK:STDOUT:   %func.decl: %func.type = fn_decl @func [concrete = constants.%func] {} {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   %Cpp.import_cpp = import_cpp {
+// CHECK:STDOUT:     import Cpp inline
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: fn @Run() {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %Cpp.ref: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
@@ -84,6 +94,12 @@ fn Run() {
 // CHECK:STDOUT:   %another_func.decl: %another_func.type = fn_decl @another_func [concrete = constants.%another_func] {} {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   %Cpp.import_cpp = import_cpp {
+// CHECK:STDOUT:     import Cpp inline
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: fn @Run() {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %Cpp.ref: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]

toolchain/sem_ir/formatter.cpp

@@ -1210,10 +1210,16 @@ auto Formatter::FormatImportCppDeclRhs() -> void {
   OpenBrace();
   for (ImportCpp import_cpp : sem_ir_->import_cpps().values()) {
     Indent();
-    out_ << "import Cpp \""
-         << FormatEscaped(
-                sem_ir_->string_literal_values().Get(import_cpp.library_id))
-         << "\"\n";
+    out_ << "import Cpp ";
+    if (import_cpp.library_id.has_value()) {
+      out_ << "\""
+           << FormatEscaped(
+                  sem_ir_->string_literal_values().Get(import_cpp.library_id))
+           << "\"";
+    } else {
+      out_ << "inline";
+    }
+    out_ << "\n";
   }
   CloseBrace();
 }