swift234

SILGen: Clean up witness thunk re-abstraction

We can re-abstract directly from the requirement signature to the
witness signature, now that re-abstraction thunks take an
abstraction pattern for both the input and the output.

To do this, we need to know the SILFunctionType of the witness
before we begin re-abstraction. Refactor getWitnessFunctionRef()
a bit to make this work.