This fixes <rdar://problem/6497650> More type mismatches issues with clang.

Move two key ObjC typechecks from Sema::CheckPointerTypesForAssignment() to ASTContext::mergeTypes().

This allows us to take advantage of the recursion in ASTContext::mergeTypes(), removing some bogus warnings.

This test case I've added includes an example where we still warn (and GCC doesn't). Need to talk with folks and decide what to do. At this point, the major bogosities should be fixed.

llvm-svn: 65231