swift234

SILGen: Allow direct calls to materializeForSet defined in a protocol extension

Now that we apply the callback with the correct generic signature, the
assert can go away. It was being triggered if the protocol extension was
defined in a different resilience domain; otherwise we prefer direct
access anyway.

Note that materializeForSet now has to be able to re-abstract Self when
invoking the callback, since we might have to go from a thin metatype
to a thick metatype.