[ms-abi] Fixes improperly sized vfptrs with pragma pack

With pragma pack, the layout engine would produce vfptrs that were 
packed width rather than pointer width.  This patch addresses the issue 
and adds a test case.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198059 91177308-0d34-0410-b5e6-96231b3b80d8