Ensure exported entities aren't remapped/duplicated (#7304)

This ensures the clang_decls map is used as a cache - without this,
visiting the same entity twice could cause it to be
re-exported/duplicated. See attached test case.

toolchain/check/testdata/interop/cpp/function/export/alias.carbon

@@ -0,0 +1,23 @@
+// 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/int.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/function/export/alias.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/export/alias.carbon
+
+// --- alias_identical_address.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+fn F();
+alias G = F;
+
+inline Cpp '''
+static_assert(&Carbon::F == &Carbon::G);
+''';