Debug info: Use the vbtable offset for virtual bases in the MS ABI

There are no vtable offset offsets in the MS ABI, but vbtable offsets
are analogous.  There are no consumers of this information yet, but at
least we don't crash now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215149 91177308-0d34-0410-b5e6-96231b3b80d8