[ms-cxxabi] Implement member pointer emission and dereferencing

Summary:
Handles all inheritance models for both data and function member
pointers.

Also implements isZeroInitializable() and refactors some of the null
member pointer code.

MSVC supports converting member pointers through virtual bases, which
clang does not (yet?) support.  Implementing that extension is covered
by http://llvm.org/15713

Reviewers: rjmccall

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D613