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

[Kees Cook via cfe-commits]

https://github.com/kees commented:

I think the docs look good, and other than what Aaron noted for needed Sema tests, the other tests look like what I'd expect.

I've run this on an updated Linux kernel with added tests for using `kcfi_salt` and CFI correctly trips.

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