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

[John McCall via cfe-commits]

rjmccall wrote:

It'll be tested in a follow-up commit which actually adds parsing and Sema support for these as qualifiers.  That should be sufficient rather than adding a whole unit-test apparatus.  Daniil is just trying to avoid landing this in a single massive commit.

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