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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I believe our feedback on the last one was to emit the namespace in all attribute ignored diagnostics, not just if we don't know the namespace.  So unknown attribute `clang::whatever` as well?

Otherwise, I think this is a reasonable patch that makes this diagnostic somewhat more clear.

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