[ms-cxxabi] Implement member pointer comparisons

Summary:
Like Itanium, comparisons are basically bitwise comparisons of the two
values, with an exception for null member function pointers.  If two
function pointers are null, only the function pointer field matters for
comparison purposes.  The rest of the bits can be arbitrary.  We take
advantage of this in isZeroInitializable(), and it may matter once we
start emitting conversions.

Reviewers: rjmccall

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D695