Only export each class or namespace to C++ once. (#7042)

Instead of exporting a class or namespace each time a new C++ name
lookup discovers it, track that we have exported the entity on its name
scope, and if a new name lookup finds the same entity, produce the same
clang declaration.

toolchain/check/testdata/interop/cpp/function/thunk_ast.carbon

@@ -12,6 +12,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/thunk_ast.carbon
 // CHECK:STDOUT: TranslationUnitDecl {{0x[a-f0-9]+}} <<invalid sloc>> <invalid sloc>
+// CHECK:STDOUT: |-NamespaceDecl {{0x[a-f0-9]+}} <<invalid sloc>> <invalid sloc> Carbon
 
 // --- thunk_required.h
 
@@ -32,6 +33,7 @@ auto foo(short a) -> void;
 // CHECK:STDOUT:   |-InternalLinkageAttr {{0x[a-f0-9]+}} <<invalid sloc>> Implicit
 // CHECK:STDOUT:   `-AsmLabelAttr {{0x[a-f0-9]+}} <col:6> Implicit "_Z3foos.carbon_thunk"
 // CHECK:STDOUT: TranslationUnitDecl {{0x[a-f0-9]+}} <<invalid sloc>> <invalid sloc>
+// CHECK:STDOUT: |-NamespaceDecl {{0x[a-f0-9]+}} <<invalid sloc>> <invalid sloc> Carbon
 
 // --- import_thunk_required.carbon
 

toolchain/check/testdata/interop/cpp/namespace/export.carbon

@@ -0,0 +1,30 @@
+// 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/none.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/namespace/export.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/namespace/export.carbon
+
+// --- alias_identity.carbon
+
+import Cpp;
+
+namespace M;
+alias N = M;
+
+class M.C {}
+
+inline Cpp '''
+// OK, valid redeclaration: M and N are the same namespace.
+namespace A = Carbon::M;
+namespace A = Carbon::N;
+
+// Valid: M::C and N::C are the same class.
+Carbon::M::C *pmc;
+Carbon::N::C *pnc = pmc;
+''';

toolchain/check/testdata/interop/cpp/namespace/import.carbon

@@ -6,9 +6,9 @@
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
-// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/namespace/basic.carbon
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/namespace/import.carbon
 // TIP: To dump output, run:
-// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/namespace/basic.carbon
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/namespace/import.carbon
 
 // ============================================================================
 // Single

toolchain/check/testdata/interop/cpp/namespace/roundtrip.carbon

@@ -81,16 +81,16 @@ library "[[@TEST_NAME]]";
 
 import Cpp;
 
-// CHECK:STDERR: fail_carbon_raw_cpp.carbon:[[@LINE+4]]:7: error: semantics TODO: `interop with non-identifier package name` [SemanticsTodo]
-// CHECK:STDERR: alias r#Cpp = Cpp;
-// CHECK:STDERR:       ^~~
-// CHECK:STDERR:
 alias r#Cpp = Cpp;
 
 inline Cpp '''
 void F();
 
 void G() {
+  // CHECK:STDERR: fail_carbon_raw_cpp.carbon:[[@LINE+8]]:16: error: semantics TODO: `interop with translation unit decl` [SemanticsTodo]
+  // CHECK:STDERR:   Carbon::Cpp::F();
+  // CHECK:STDERR:                ^
+  // CHECK:STDERR:
   // CHECK:STDERR: fail_carbon_raw_cpp.carbon:[[@LINE+4]]:11: error: no member named 'Cpp' in namespace 'Carbon' [CppInteropParseError]
   // CHECK:STDERR:    19 |   Carbon::Cpp::F();
   // CHECK:STDERR:       |   ~~~~~~~~^

toolchain/check/testdata/interop/cpp/reverse/class.carbon

@@ -107,3 +107,17 @@ inline Cpp '''
 // CHECK:STDERR:
 static_assert(sizeof(Carbon::A) == 2 * sizeof(int));
 ''';
+
+// --- alias_identity.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+class A {}
+alias B = A;
+
+inline Cpp '''
+// OK, Carbon::A and Carbon::B are the same type.
+Carbon::A *pa;
+Carbon::B *pb = pa;
+''';