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

[Bill Wendling via cfe-commits]

bwendling wrote:

@erichkeane If the Arm attribute code were merged into `ExtraAttributeInfo`, it would save us a bit in `FunctionTypeExtraBitfields`.

I can add more testing I suppose, but the tests I have here are a copy of the current CFI tests, which test all of the ways CFI is used. So I'm not sure how much more needs to be done... The important thing that's tested here is that the CFI code changes in a deterministic way when the "salt" is added. It's not a test for all of CFI.

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