MS ABI: Up the required alignment after inserting padding between vbases

We would correctly insert sufficiently aligned padding between vbases
when our leading base was empty, however we would neglect to increase
the required alignment of the most derived class.

This fixes PR20315.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@213123 91177308-0d34-0410-b5e6-96231b3b80d8