[clang] [llvm] [X86] Extend kCFI with a 3-bit arity indicator (PR #121070)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> maurer: I think their point is that even if you are not changing the hash scheme, you are proposing breaking compatibility of the identifier with existing code. Since we don't want to do this many times, if we are breaking compatibility with existing code, they would like to batch it with another breaking update so that it doesn't need to be done again.
>
> (This isn't me reviewing this PR, just trying to clear up some confusion.)

This:) Thanks for the explanation.

After my pending lld/MachO change, kcfi will be the only user of the legacy `xxHash64`. I want to remove `xxHash64` from llvm-project.

https://github.com/llvm/llvm-project/pull/121070