[clang] [clang][frontend] Add support for attribute plugins for statement attributes (PR #110334)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

This seems reasonable?  I actually didn't know we weren't letting them create new attributes, and were just using `AnnotateAttr` in the AST.  I think @AaronBallman should do 1 run through this as he understands the plugins better than I, but this otherwise LGTM.

https://github.com/llvm/llvm-project/pull/110334