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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I think this would be a great opportunity to save a bit of memory on `AttributedType`, else this is probably ok to me.

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