[clang] [Clang][attr] Add 'cfi_salt' attribute (PR #141846)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

A few small things, but this is going to be LGTM pretty quickly.

Approve pending my 4 comments.

https://github.com/llvm/llvm-project/pull/141846