[clang] [Clang] add fix-it hints for unknown attributes (PR #141305)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I'm really in favor of this direction, this is great, thank you!

I have a few suggestions here (and if there is good reason, feel free to ignore the ones re return-type & setting invalid in particular), but this is pretty good for me.

https://github.com/llvm/llvm-project/pull/141305