swift234

IRGen: Update calling convention for generic witnesses.

Have WitnessBuilder awkwardly ask SIL's TypeConverter to uncurry witness types, so that polymorphic arguments get emitted in the right order consistent with lowered SILFunctions, instead of doing the uncurrying itself directly from the Swift type. This is layer-violating as hell but gets generic witness codegen working. I have a feeling SILGen will want to take over witness thunk emission at some point.

Swift SVN r6401