[clang] [clang][PAC] ptrauth_qualifier and ptrauth_intrinsic should only be available on Darwin (PR #153912)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin commented:

Can you add a blurb in the release note saying _has_feature(ptrauth_qualifier) is deprecated and __PTRAUTH__ should be used instead?

https://github.com/llvm/llvm-project/pull/153912