swift234

SILGen: Prepare witness thunk emission for default witnesses, NFC

In this case we do not have a conformance, and the default witness
thunk uses the same signature and context archetypes as the protocol
requirement.

There might still be an abstraction change between the requirement
and witness, though.

Tests are in the next patch that actually adds the ability to emit
these thunks. This is just a refactoring.