[clang] [clang][PAC][NFC] Provide addition support macros to ptrauth.h (PR #161027)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> @ojhunt Can we consider this PR obsolete and close it since we no longer use this union approach and things are already (almost) fixed in the main unwinding PR #143230?

They're macros that are used, and are useful on their own so upstreaming still seems sensible? they were just for the unwind work I had just genuinely assumed they were already upstreamed :D

https://github.com/llvm/llvm-project/pull/161027