[clang] [Clang] Preserve namespace identity for inline namespace redeclarations (PR #233862)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

Also, this PR is close enough to https://cplusplus.github.io/CWG/issues/2061.html that it sounds reasonable to add a C++ DR test for it. If you're new to C++ DR tests, we have documentation about them: https://clang.llvm.org/docs/InternalsManual.html#c-defect-report-tests

https://github.com/llvm/llvm-project/pull/233862