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

[Anton Korobeynikov via llvm-commits]

asl wrote:

@nikic Will you please take another look when you will have a chance? This PR is currently blocker for many frontend and backend-related pauth changes.

Thanks!

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