[MS-ABI] Fix alias-avoidance padding between bases

Adds padding between bases or virtual bases in an attempt to avoid 
aliasing of zero-sized sub-objects.  The approach used by the ABI adds 
two more bits of state.  Detailed comments are in the code.  Test cases 
included.

Differential Revision: http://llvm-reviews.chandlerc.com/D2258



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196602 91177308-0d34-0410-b5e6-96231b3b80d8