[ms-cxxabi] Move MS inheritance model calculation into MemberPointerType

Summary:
This makes it possible to share code between lib/AST/MicrosoftCXXABI.cpp
and lib/CodeGen/MicrosoftCXXABI.cpp.  No functionality change.

Also adds comments about the layout of the member pointer structs as I
currently understand them.

Reviewers: rjmccall

CC: timurrrr, cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D590