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

This brings declarations of `PointerAuthQualifier` class and
`PointerAuthenticationMode` enum and related functions required for
PAuth support in lldb (see #84387) from downstream Apple's code. See
#84387 for tests as well.

Co-authored-by: Ahmed Bougacha <ahmed@bougacha.org>
Co-authored-by: John McCall <rjmccall@apple.com>

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Co-authored-by: John McCall <rjmccall@apple.com>
Co-authored-by: Ahmed Bougacha <ahmed@bougacha.org>