MS ABI: Tweak pointer-to-member mangling/inheritance model selection

Properly determine the inheritance model when dealing with nullptr:
- If a nullptr template argument is being checked against
  pointer-to-member parameter, nail down an inheritance model.
  N.B. We will chose an inheritance model even if we won't ultimately
  choose the template to instantiate!  Cooky, right?
- Null pointer-to-datamembers have a virtual base table offset of -1,
  not zero. Previously, we chose an offset of 0.