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

[Bill Wendling via cfe-commits]

bwendling wrote:

> Maybe nice to call it kcfi_sault?
> 
> -fsanitize=kcfi is completely different from -fsanitize=cfi, so it would be consistent to keep the convention in attributes as well.

A few people suggested naming it `cfi_salt` because it could be used in other contexts:

https://github.com/llvm/llvm-project/pull/141846#issuecomment-3085340995
https://github.com/llvm/llvm-project/pull/141846#issuecomment-3085354901

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