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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Testing seems insufficient, particularly around conversion rules/etc of these types.  I would like to see quite a bit more for Sema/inside of templates/how this applies to instantiations of all these things, etc.

Also, this is ANOTHER bit on function types that we're using for CFI.  I'm wondering if this is actually a generally useful enough feature to be using this much compiler resources.

@AaronBallman ?

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