[clang] [clang/AST] Make it possible to use SwiftAttr in type context (PR #108631)

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun approved this pull request.

Overall looks good to me, I have one question and a couple nits inline. I think having access to Attr from AttributedType makes sense, I would not be surprised if this helped some other use cases like Lifetime annotations in Crubit. But please wait for a review from @erichkeane as well. 

https://github.com/llvm/llvm-project/pull/108631