[ms-abi] Adjusting Rules for Padding Between Bases

The presence of a VBPtr suppresses the presence of zero sized 
sub-objects in the non-virtual portion of the object in the context of 
determining if two base objects need alias-avoidance padding placed 
between them.

Test cases included.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198975 91177308-0d34-0410-b5e6-96231b3b80d8