MS ABI: Add support for #pragma pointers_to_members

Introduce a notion of a 'current representation method' for
pointers-to-members.

When starting out, this is set to 'best case' (representation method is
chosen by examining the class, selecting the smallest representation
that would work given the class definition or lack thereof).

This pragma allows the translation unit to dictate exactly what
representation to use, similar to how the inheritance model keywords
operate.

N.B.  PCH support is forthcoming.

Differential Revision: http://llvm-reviews.chandlerc.com/D2723