[llvm] [PAC][IR][AArch64] Add "ptrauth(...)" Constant to represent signed pointers. (PR #85738)

[Ahmed Bougacha via llvm-commits]

ahmedbougacha wrote:

FWIW, RFC here:
  https://discourse.llvm.org/t/rfc-adding-ptrauth-constants/78926

I was waiting to also bring up the bundles there (from https://github.com/llvm/llvm-project/pull/85736), but on second thought those aren't really interesting enough (at the IR level, beyond backend boilerplate) to be deserving of that; let's focus on constants!

https://github.com/llvm/llvm-project/pull/85738