[AArch64][PAC] Support ptrauth builtins and -fptrauth-intrinsics. (#65996)

This defines the basic set of pointer authentication clang builtins
(provided in a new header, ptrauth.h), with diagnostics and IRGen
support.  The availability of the builtins is gated on a new flag,
`-fptrauth-intrinsics`.

Note that this only includes the basic intrinsics, and notably excludes
`ptrauth_sign_constant`, `ptrauth_type_discriminator`, and
`ptrauth_string_discriminator`, which need extra logic to be fully
supported.

This also introduces clang/docs/PointerAuthentication.rst, which
describes the ptrauth model in general, in addition to these builtins.

Co-Authored-By: Akira Hatanaka <ahatanaka@apple.com>
Co-Authored-By: John McCall <rjmccall@apple.com>