[clang] [llvm] [CodeGen][AArch64] ptrauth intrinsic to safely construct relative ptr (PR #142047)

[John McCall via cfe-commits]

https://github.com/rjmccall requested changes to this pull request.

Oliver is right that we need custom checking in SemaChecking akin to the other builtins, e.g. to validate that the keys are constants. The pattern should be straightforward to follow. Otherwise the frontend changes LGTM except as noted.

https://github.com/llvm/llvm-project/pull/142047