Support round-tripping entities through C++ and Carbon. (#7022)

Use the same C++ -> Carbon map for both interop directions, and when
importing an entity from Carbon -> C++, check whether it was originally
a C++ entity and if so return the original.

Assisted-by: Gemini via Google Antigravity

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

@@ -0,0 +1,43 @@
+// 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/class/roundtrip.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/class/roundtrip.carbon
+
+// --- carbon_to_cpp_to_carbon.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+class C {}
+
+inline Cpp '''
+using CAlias = Carbon::C;
+''';
+
+fn F(p: C*) {
+  let unused q: Cpp.CAlias* = p;
+}
+
+// --- cpp_to_carbon_to_cpp.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+inline Cpp '''
+class C {};
+''';
+
+alias CAlias = Cpp.C;
+
+inline Cpp '''
+auto F(C *p) -> Carbon::CAlias* {
+  return p;
+}
+''';