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

[Bill Wendling via cfe-commits]

bwendling wrote:

I moved the `cfi_salt` into the `FunctionProtoType` fields. I left it open for further additions. One bit of optimization I could do is merge the Arm stuff into this new struct, but wanted to do something a bit less cluttered first.

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