[AArch64] __builtin_return_address for PAuth.

This change adds the support for __builtin_return_address
for ARMv8.3A Pointer Authentication.
Location of the authentication code in the pointer depends on
the system configuration, therefore a dedicated instruction is used for
effectively removing the authentication code without
authenticating the pointer.

Reviewed By: chill

Differential Revision: https://reviews.llvm.org/D75044