[clang] Raise an error on namespace aliases with qualified names. (PR #86122)

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https://github.com/Sirraide requested changes to this pull request.

Thanks for contributing! This needs a release note (in `clang/docs/ReleaseNotes.rst`) and some changes to the diagnostics, but other than that it looks fine to me.

https://github.com/llvm/llvm-project/pull/86122