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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

So I think this patch is doing too much, I'd like to see the changes to `AttributedType` split into a separate patch so we can talk about the `swift_attr` alone.

As far as moving this attribute to be a Type Attribute, that comes with a ton of concerns around templates.  I don't see anything set to instantiate teh arguments, so any value you give to it will not be instantiated (the attribute will as far as I can tell, be instantiated, but with no modifications to its arguments).

I would like to see a some tests (AST and not!) that validate the behavior of this on a template, and as a type being instantiated, to ensure it doesn't get lost in cases where you'd want it to stay.

Also, we need a release note.

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