[MS ABI] Account for the virtual inheritance quirk when mangling

Virtual inheritance member pointers are always relative to the vbindex,
even when the member pointer doesn't point into a virtual base.  This is
corrected by adjusting the non-virtual offset backwards from the vbptr
back to the top of the most derived class.  While we performed this
adjustment when manifesting member pointers as constants or when
performing conversions, we didn't perform the adjustment when mangling
them.

llvm-svn: 240453