[clang] Define ptrauth_string_discriminator builtin.

This exposes the ABI-stable hash function that allows computing a 16-bit
discriminator from a constant string.

This allows manually matching the implicit string discriminators
computed in the ABI (e.g., from mangled names for vtable pointer/entry
signing), as well as enabling the use of interesting discriminators when
manually annotating specific pointers with the __ptrauth qualifier.

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