swift234

SILGen: Emit vtable thunks to handle optional variance.

If a subclass overrides methods with variance in the optionality of non-class-type members, emit a thunk to handle wrapping more optional parameters or results and force-unwrapping any IUO parameters made non-optional in the derived. For this to be useful, we need IRGen to finally pay attention to SILVTables, but this is a step on the way to fixing rdar://problem/19321484.

Swift SVN r24705