swift234

SILGen: Targeted fix for MaterializeForSet use

If we're accessing a field of a struct, ignore materializeForSet;
it will only have been synthesized if the struct conforms to a
protocol, and we don't want the presence of a conformance to
affect generated code.

A more principled fix would change the SILGen logic to use a
materializeForSet if it would have been synthesized anyway,
asserting that it was synthesized in that case. I'll clean this
up in the 'master' branch once these fixes settle.