Fixing PR18430 by checking that the size of bitfields plus padding does not
grow into the following virtual base.

Differential Revision: http://llvm-reviews.chandlerc.com/D2560



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200359 91177308-0d34-0410-b5e6-96231b3b80d8