- Follow C99 behavior of using other operand type when one of
   operands is a null pointer constant.
 - Fix overenthusiastic devolving of any Objective-C types to id:
   o If either operand has an Objective-C object type then:
     - If both operands are interfaces and either operand can be
       assigned to the other, use that type as the composite type.
     - Otherwise, if either type is id, use id as the composite type.
     - Otherwise, warn about incompatible types and use id as the
       composite type.
 - Return handling of qualified idea to separate test following
   general pointer type checking.
   o Upgraded from old code to allow devolving to id (without warning,
     which matches GCC).
 - <rdar://problem/6212771>

Add test case for issues fixed above, XFAIL though because it exposed
a new issue in property handling.

llvm-svn: 56135