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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> 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 ?

I think CFI is pretty important for security posture (it basically tries to ensure that indirect jumps don't go to unintended locations). So I think it's useful, but I am also a bit worried that we're missing the big picture of how much work needs to be done to fully support CFI. @bwendling do you expect to land more related attributes after this one?

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