[clang] [Clang] diagnose unknown attribute namespaces (PR #138519)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

So I don't believe this is correct either.  We want it to always say the namespace.  So `clang::unknown` will diagnose as `unknown attribute 'clang::unknown' ignored` AFAIK.  Right @AaronBallman ?

https://github.com/llvm/llvm-project/pull/138519