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/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;
+''';