[ObjC] Add some additional test cases around pointer conversions.
This is especially important for Objective-C++, which is entirely
missing this testing at the moment.
This annotates with "FIXME" the cases which I change in the next
patch -- I primarily wanted to document the current state of things so
that the effect of the code change is made clear.
Differential Revision: https://reviews.llvm.org/D67982