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

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

> libstdc++ [declares _Dir inside its C++11 ABI namespace](https://github.com/gcc-mirror/gcc/blob/releases/gcc-14.2.0/libstdc%2B%2B-v3/include/bits/fs_dir.h#L94-L98), then [defines it as fs::_Dir through the std::filesystem alias](https://github.com/gcc-mirror/gcc/blob/releases/gcc-14.2.0/libstdc%2B%2B-v3/src/c%2B%2B17/fs_dir.cc#L39-L45).

Thank you for the pointers, but can you make a small representative example of what libstdc++ does, or point out to an existing test? It's not 100% clear what `_GLIBCXX_BEGIN_NAMESPACE_CXX11` expands into, and I don't want to spend too much time in GPL code chasing things down.

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