[MS ABI] Correctly make paths through covariant virtual bases

There can be multiple virtual bases which are on the path to a vfptr
when one vbase virtually inherits from another.  We should prefer the
most derived virtual base which covariantly overrides a method in the
vfptr class;  if we do not lengthen the path this way, we will end up
with too few vftable entries.

This fixes PR21073.

llvm-svn: 236239