[MS] Improve VPtrInfo field names and doc comments

'ReusingBase' was a terrible name. It might actually refer to the most
derived class, which is not a base. 'BaseWithVPtr' was also bad, since
again, it could refer to the most derived class. It was actually the
first base to introduce the vptr, so now it is 'IntroducingObject'.

llvm-svn: 276120