DWARF type hashing: pointers to members

Includes a test case/FIXME demonstrating a bug/limitation in pointer to
member hashing. To be honest I'm not sure why we don't just always use
summary hashing for referenced types... but perhaps I'm missing
something.
Use GEPs correctly when adjusting this in MicrosoftCXXABI

Reviewed at http://llvm-reviews.chandlerc.com/D1977