[MS ABI] Don't crash when zero-initializing a vbase which contains a vbase

Bases can be zero-initialized: the storage is zero-initialized before
the base constructor is run.
The MS ABI has a quirk where base VBPtrs are not installed by the
base constructor but by the most derived class.  In particular, they are
installed before the base constructor is run.
The derived constructor must be careful to zero-initialize only the bits
of the class which haven't already been populated by virtual base
pointers.

While we correctly avoided this scenario, we didn't handle the case
where the base class has virtual bases which have virtual bases.

llvm-svn: 269271