swift234

SILGen: Factor out TypeConverter::getMaterializeForSetCallbackType(), NFC

I'm planning on changing materializeForSet to return the callback
as a RawPointer, and casting it to the correct polymorphic SILFunctionType
type at the call site.

This will allow us to call it with the right generic signature, which
is currently not possible since it is returned as an optional of an
AST function type, and optionals of SIL function types are not permitted.