swift234

SILGen: Refactoring MaterializeForSet to support default witness thunk emission

Previously we would emit two types of MaterializeForSet implementations
in SILGen:

- materializeForSet for a concrete storage declaration

- materializeForSet witness thunk in a conformance

This refactoring decouples the code from taking a conformance, which is
needed for two new types of materializeForSet that we need:

- materializeForSet witness thunk in a default witness table -- this is
  necessary in order to be able to resiliently add storage requirements
  with default implementations to protocols

- materializeForSet vtable thunk -- this is necessary to fix a missing
  re-abstraction case with overriding storage in a subclass

This patch brings us closer to implementing these two. For default
implementations, we still have an issue in that the materializeForSet
has a different "generic signature abstraction pattern" in concrete
and default witnesses, so default and concrete witnesses for
materializeForSet are currently ABI-incompatible because the type
metadata for the storage is passed differently to the callback.