[ms-cxxabi] Implement member pointer conversions

Summary:
This only supports converting along non-virtual inheritance paths by
changing the field offset or the non-virtual base adjustment.

This implements three kinds of conversions:
- codegen for Value conversions
- Constant emission for APValue
- Constant folding for CastExprs

In almost all constant initialization settings
EmitMemberPointer(APValue) is called, except when the expression
contains a reinterpret cast.

reinterpret casts end up being a big corner case because the null value
changes between different kinds of member pointers.

Reviewers: rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D741