[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

llvm-svn: 375124