Fix the field count in interface record layout (it was incorrectly

compensating for super classes). This was making the reported class
sizes for empty classes very, very wrong.
 - Also, we now report the size info for an empty class like gcc (as
   the offset of the start, not as 0, 0).

 - Add a few more test cases we were mishandling before (padding bit
   field at end of struct, for example).

llvm-svn: 70938