C++ interop: add a simple unit test for "using enum" (#6278)

This unit test uses "using UEnum = Enum;" and imports that into Carbon.

This is my very first try at contributing something to carbon, I'm
looking forward to your feedback.
The test is very basic. 
What other cases should it test ?
What other comments do you have ?

toolchain/check/testdata/interop/cpp/enum/using.carbon

@@ -0,0 +1,138 @@
+// 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
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/uint.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/enum/using.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/enum/using.carbon
+
+// --- enum.h
+
+
+enum class Color {Red, Blue, Green};
+
+using enum Color;
+
+class Zoo {
+public:
+  enum class Animal { Cat, Dog, Horse };
+};
+
+using enum Zoo::Animal;
+
+// --- import_enum.carbon
+
+library "[[@TEST_NAME]]";
+
+// CHECK:STDERR: import_enum.carbon:[[@LINE+10]]:10: in file included here [InCppInclude]
+// CHECK:STDERR: ./enum.h:5:7: warning: using enum declaration is a C++20 extension [CppInteropParseWarning]
+// CHECK:STDERR:     5 | using enum Color;
+// CHECK:STDERR:       |       ^
+// CHECK:STDERR:
+// CHECK:STDERR: import_enum.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
+// CHECK:STDERR: ./enum.h:12:7: warning: using enum declaration is a C++20 extension [CppInteropParseWarning]
+// CHECK:STDERR:    12 | using enum Zoo::Animal;
+// CHECK:STDERR:       |       ^
+// CHECK:STDERR:
+import Cpp library "enum.h";
+
+//@dump-sem-ir-begin
+fn F() {
+  let col : Cpp.Color = Cpp.Color.Red;
+  let colb : Cpp.Color = Cpp.Blue;
+  let pet : Cpp.Zoo.Animal = Cpp.Zoo.Animal.Cat;
+  let pet2 : Cpp.Zoo.Animal = Cpp.Dog;
+}
+//@dump-sem-ir-end
+
+// CHECK:STDOUT: --- import_enum.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
+// CHECK:STDOUT:   %Color: type = class_type @Color [concrete]
+// CHECK:STDOUT:   %pattern_type.839: type = pattern_type %Color [concrete]
+// CHECK:STDOUT:   %int_0.40a: %Color = int_value 0 [concrete]
+// CHECK:STDOUT:   %int_1.a8e: %Color = int_value 1 [concrete]
+// CHECK:STDOUT:   %Zoo: type = class_type @Zoo [concrete]
+// CHECK:STDOUT:   %Animal: type = class_type @Animal [concrete]
+// CHECK:STDOUT:   %pattern_type.d9d: type = pattern_type %Animal [concrete]
+// CHECK:STDOUT:   %int_0.83f: %Animal = int_value 0 [concrete]
+// CHECK:STDOUT:   %int_1.afa: %Animal = int_value 1 [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Cpp: <namespace> = namespace file.%Cpp.import_cpp, [concrete] {
+// CHECK:STDOUT:     .Color = %Color.decl
+// CHECK:STDOUT:     .Blue = %int_1.a8e
+// CHECK:STDOUT:     .Zoo = %Zoo.decl
+// CHECK:STDOUT:     .Dog = %int_1.afa
+// CHECK:STDOUT:     import Cpp//...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Color.decl: type = class_decl @Color [concrete = constants.%Color] {} {}
+// CHECK:STDOUT:   %int_0.40a: %Color = int_value 0 [concrete = constants.%int_0.40a]
+// CHECK:STDOUT:   %int_1.a8e: %Color = int_value 1 [concrete = constants.%int_1.a8e]
+// CHECK:STDOUT:   %Zoo.decl: type = class_decl @Zoo [concrete = constants.%Zoo] {} {}
+// CHECK:STDOUT:   %Animal.decl: type = class_decl @Animal [concrete = constants.%Animal] {} {}
+// CHECK:STDOUT:   %int_0.83f: %Animal = int_value 0 [concrete = constants.%int_0.83f]
+// CHECK:STDOUT:   %int_1.afa: %Animal = int_value 1 [concrete = constants.%int_1.afa]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   name_binding_decl {
+// CHECK:STDOUT:     %col.patt: %pattern_type.839 = value_binding_pattern col [concrete]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Cpp.ref.loc18_25: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %Color.ref.loc18_28: type = name_ref Color, imports.%Color.decl [concrete = constants.%Color]
+// CHECK:STDOUT:   %Red.ref: %Color = name_ref Red, imports.%int_0.40a [concrete = constants.%int_0.40a]
+// CHECK:STDOUT:   %.loc18: type = splice_block %Color.ref.loc18_16 [concrete = constants.%Color] {
+// CHECK:STDOUT:     %Cpp.ref.loc18_13: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:     %Color.ref.loc18_16: type = name_ref Color, imports.%Color.decl [concrete = constants.%Color]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %col: %Color = value_binding col, %Red.ref
+// CHECK:STDOUT:   name_binding_decl {
+// CHECK:STDOUT:     %colb.patt: %pattern_type.839 = value_binding_pattern colb [concrete]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Cpp.ref.loc19_26: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %Blue.ref: %Color = name_ref Blue, imports.%int_1.a8e [concrete = constants.%int_1.a8e]
+// CHECK:STDOUT:   %.loc19: type = splice_block %Color.ref.loc19 [concrete = constants.%Color] {
+// CHECK:STDOUT:     %Cpp.ref.loc19_14: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:     %Color.ref.loc19: type = name_ref Color, imports.%Color.decl [concrete = constants.%Color]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %colb: %Color = value_binding colb, %Blue.ref
+// CHECK:STDOUT:   name_binding_decl {
+// CHECK:STDOUT:     %pet.patt: %pattern_type.d9d = value_binding_pattern pet [concrete]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Cpp.ref.loc20_30: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %Zoo.ref.loc20_33: type = name_ref Zoo, imports.%Zoo.decl [concrete = constants.%Zoo]
+// CHECK:STDOUT:   %Animal.ref.loc20_37: type = name_ref Animal, imports.%Animal.decl [concrete = constants.%Animal]
+// CHECK:STDOUT:   %Cat.ref: %Animal = name_ref Cat, imports.%int_0.83f [concrete = constants.%int_0.83f]
+// CHECK:STDOUT:   %.loc20: type = splice_block %Animal.ref.loc20_20 [concrete = constants.%Animal] {
+// CHECK:STDOUT:     %Cpp.ref.loc20_13: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:     %Zoo.ref.loc20_16: type = name_ref Zoo, imports.%Zoo.decl [concrete = constants.%Zoo]
+// CHECK:STDOUT:     %Animal.ref.loc20_20: type = name_ref Animal, imports.%Animal.decl [concrete = constants.%Animal]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %pet: %Animal = value_binding pet, %Cat.ref
+// CHECK:STDOUT:   name_binding_decl {
+// CHECK:STDOUT:     %pet2.patt: %pattern_type.d9d = value_binding_pattern pet2 [concrete]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Cpp.ref.loc21_31: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %Dog.ref: %Animal = name_ref Dog, imports.%int_1.afa [concrete = constants.%int_1.afa]
+// CHECK:STDOUT:   %.loc21: type = splice_block %Animal.ref.loc21 [concrete = constants.%Animal] {
+// CHECK:STDOUT:     %Cpp.ref.loc21_14: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:     %Zoo.ref.loc21: type = name_ref Zoo, imports.%Zoo.decl [concrete = constants.%Zoo]
+// CHECK:STDOUT:     %Animal.ref.loc21: type = name_ref Animal, imports.%Animal.decl [concrete = constants.%Animal]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %pet2: %Animal = value_binding pet2, %Dog.ref
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT: