swift234

SILGen: Cast the materializeForSet callback to the correct type when applying it

We don't want to perform substitutions when we call the materializeForSet
accessor itself, since the return value is a polymorphic thin function,
and its calling convention is not compatible with a concretely-typed
function value in the case where 'Self' is an abstract type parameter.

With this change, the materializeForSet declaration still has an AST type
for the callback in its return value, but since this AST type makes no
sense in reality it would be better to just return a RawPointer instead,
removing some unnecessary code from CodeSynthesis.cpp. This will be
cleaned up in a subsequent patch.