[clang] [PAC][clang] Define `PointerAuthQualifier` and `PointerAuthenticationMode` (PR #84384)

[Michael Buch via cfe-commits]

Michael137 wrote:

LGTM, but would like @rjmccall or @AaronBallman (or another code-owner) to give final approval

https://github.com/llvm/llvm-project/pull/84384