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

[Aaron Ballman via cfe-commits]

AaronBallman 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` AFAIK. Right @AaronBallman ?

I think what I'm seeing is correct, unless I'm missing something. Attributes without namespaces are printed as `foo` and attributes with namespaces are now being printed as `bar::foo` instead of `foo`.

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