[llvm] [PAC][AArch64] Lower ptrauth constants in code (PR #96879)

[Ahmed Bougacha via llvm-commits]

https://github.com/ahmedbougacha approved this pull request.

TBH I don't know that `Mask16` helps in the first place, given that `0xFFFF` is immediately recognizable in a way no named variable could ever be.  But I had the same thought in #94241;  at the end of the day I suppose it's subjective, I don't mind either way, so LGTM

https://github.com/llvm/llvm-project/pull/96879