[AArch64][PAC] Lower authenticated calls with ptrauth bundles. (#85736)

This adds codegen support for the "ptrauth" operand bundles, which can
be used to augment indirect calls with the equivalent of an
`@llvm.ptrauth.auth` intrinsic call on the call target (possibly
preceded by an `@llvm.ptrauth.blend` on the auth discriminator if
applicable.)

This allows the generation of combined authenticating calls
on AArch64 (in the BLRA* PAuth instructions), while avoiding
the raw just-authenticated function pointer from being
exposed to attackers.

This is done by threading a PtrAuthInfo descriptor through
the call lowering infrastructure, eventually selecting a BLRA
pseudo.  The pseudo encapsulates the safe discriminator
computation, which together with the real BLRA* call get emitted
in late pseudo expansion in AsmPrinter.

Note that this also applies to the other forms of indirect calls,
notably invokes, rvmarker, and tail calls.  Tail-calls in particular
bring some additional complexity, with the intersecting register
constraints of BTI and PAC discriminator computation.
However this doesn't currently support PAuth_LR tail-call variants.

This also adopts an x8+ allocation order for GPR64noip, matching
GPR64.