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

[Ahmed Bougacha via llvm-commits]

ahmedbougacha wrote:

FWIW I picked this up to rebase the big ptrauth branch on top of, and I do get:
```
warning: lambda capture 'Mask16' is not required to be captured for this use [-Wunused-lambda-capture]
```

that's on a ~year-ish old clang

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