Currently we forcefully complete C++ types if we can't find their
definition for layout purposes. This ensures that we at least don't
crash in Clang when laying out the type. The definition is required for
types of members/array elements/base classes for the purposes of
calculating their layout. This is also true for Obj-C types, but we
haven't been forcefully completing those.
The test-case that's being un-XFAILed in this patch demonstrates a case
where not completing the super-class forcefully causes a clang crash.
rdar://168440264
3aea01b271