MS ABI: Include the vfptr offset in memptrs to virtual methods

Virtual methods expect 'this' to point to the vfptr containing the
virtual method, and this extends to virtual member pointer thunks.  The
relevant vfptr is always at offset zero on entry to the thunk, and no
this adjustment is needed.

Previously we would not include the vfptr adjustment in the member
pointer, and we'd look at the vfptr offset when loading from the vftable
in the thunk.

Fixes PR18917.