[AArch64][PAC] Group arguments of emitPtrauthAuthResign (NFC)
The caller of `AArch64AsmPrinter::emitPtrauthAuthResign` has to analyze
the operands of MachineInstr being emitted and pass them explicitly to
this method, which leads to large number of function arguments, some of
them being optional.
This commit introduces `struct PtrAuthSchema` to pass semantically-
related parameters as a single argument and to better express the idea
that the second schema can only be passed or omitted as a whole.
Furthermore, `AUTVal` argument is renamed to `Pointer`, as unlike other
arguments with the `AUT` prefix, it does not relate to the authentication
schema, but represents a tied in-out operand used throughout the entire
expanded instruction sequence.