[clang] [clang][PAC] ptrauth_qualifier must be considered a feature (PR #153291)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

@efriedma-quic @AaronBallman I just posted a proof of concept version of a restricted "`__has_extension` that works under -pedantic-errors" : https://github.com/llvm/llvm-project/pull/153506

(Side note, it might be nice to be able to create a way to deprecate __has_feature tests so that code using __has_feature can be warned and migrate to __has_extension)

https://github.com/llvm/llvm-project/pull/153291