There were a few problems here with subclasses of Objective-C classes.
Use the InstanceStart field from rodata to correctly lay out instance
variables, and verify the results match with dynamic and static layout.
Better fix for <rdar://problem/27932061>.
@slava_pestov recently folded in @objc classes when building class field
descriptors - we just need to update the switch when considering records
for converting TypeRefs to TypeInfos.
rdar://problem/26594130
