[clang] [clang][ObjC][PAC] Add ptrauth protections to objective-c (PR #147899)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

I'm obviously not qualified to review this but
 - Do we need to update https://clang.llvm.org/docs/PointerAuthentication.html ?
 - Do we need a changelog entry?

https://github.com/llvm/llvm-project/pull/147899