[ms-abi] Report VBPtrOffset correctly

Although VBPtrs were being placed correctly by the ms-abi layout engine, 
their offsets were being improperly reported to the ASTRecordLayout 
builder due to a bug.  This patch fixes that and fixes the test cases to 
use the correct values.
y


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199168 91177308-0d34-0410-b5e6-96231b3b80d8