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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> No, ptrauth is not a feature, it is an extension and should be reported as such. The real bug here is that you're missing an extension warning when `__ptrauth` is used

I thought our general policy was that we don't trigger pedantic warnings for reserved keywords (like anything with a `__` prefix)?

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