Microsoft adds weird padding before virtual bases if the last field was a bitfield

This patch adds the same behavior to clang.

Differential Revision: http://llvm-reviews.chandlerc.com/D2103



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194115 91177308-0d34-0410-b5e6-96231b3b80d8