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

[Oleksandr T. via cfe-commits]

a-tarasyuk wrote:

> 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 

@erichkeane @AaronBallman Thanks for the feedback. I've made changes to include the _scope_ in the diagnostic.

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