[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

llvm-svn: 196602