[ms-cxxabi] Implement member data pointers for non-dynamic classes

Summary:
For non-dynamic classes (no virtual bases), member data pointers are
simple offsets from the base of the record.  Dynamic classes use an
aggregate for member data pointers and are therefore currently
unsupported.

Unlike Itanium, the ms ABI uses 0 to represent null for polymorphic
classes.  Non-polymorphic classes use -1 like Itanium, since 0 is a
valid field offset.

Reviewers: rjmccall

CC: timurrrr, cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D558