[MS ABI] Rework member pointer conversion

Member pointers in the MS ABI are made complicated due to the following:
- Virtual methods in the most derived class (MDC) might live in a
  vftable in a virtual base.
- There are four different representations of member pointer: single
  inheritance, multiple inheritance, virtual inheritance and the "most
  general" representation.
- Bases might have a *more* general representation than classes which
  derived from them, a most surprising result.

We believed that we could treat all member pointers as-if they were a
degenerate case of the multiple inheritance model.  This fell apart once
we realized that implementing standard member pointers using this ABI
requires referencing members with a non-zero vbindex.

On a bright note, all but the virtual inheritance model operate rather
similarly.  The virtual inheritance member pointer representation
awkwardly requires a virtual base adjustment in order to refer to
entities in the MDC.

However, the first virtual base might be quite far from the start of the
virtual base.  This means that we must add a negative non-virtual
displacement.

However, things get even more complicated.  The most general
representation interprets vbindex zero differently from the virtual
inheritance model: it doesn't reference the vbtable at all.

It turns out that this complexity can increase for quite some time:
consider a derived to base conversion from the most general model to the
multiple inheritance model...

To manage this complexity we introduce a concept of "normalized" member
pointer which allows us to treat all three models as the most general
model.  Then we try to figure out how to map this generalized member
pointer onto the destination member pointer model.  I've done my best to
furnish the code with comments explaining why each adjustment is
performed.

This fixes PR23878.

llvm-svn: 240384