[IR][AArch64][PAC] Add "ptrauth(...)" Constant to represent signed pointers. (#85738)

This defines a new kind of IR Constant that represents a ptrauth signed
pointer, as used in AArch64 PAuth.

It allows representing most kinds of signed pointer constants used thus
far in the llvm ptrauth implementations, notably those used in the
Darwin and ELF ABIs being implemented for c/c++.  These signed pointer
constants are then lowered to ELF/MachO relocations.

These can be simply thought of as a constant `llvm.ptrauth.sign`, with
the interesting addition of discriminator computation: the `ptrauth`
constant can also represent a combined blend, when both address and
integer discriminator operands are used.  Both operands are otherwise
optional, with default values 0/null.