[clang] Define ptrauth_string_discriminator builtin. (#93903)

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.

The argument must be a string literal of char character type.  The
result has type ptrauth_extra_data_t.
The result value is never zero and always within range for both the
__ptrauth qualifier and ptrauth_blend_discriminator.
This can be used in constant expressions.

Co-authored-by: John McCall <rjmccall@apple.com>

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