[ms-abi] Leading VFPtrs don't suppress the leading zero sized flag

The MS-ABI tracks a bit that asserts that the first sub-object is zero 
sized.  This bit is used to add padding between objects if there's the 
potential for zero sized objects to alias.  The bit is still true even 
if the zero sized base is lead by a VFPtr.  This patch makes clang mimic 
that behavior.

llvm-svn: 199132