[Checkers] PolymorphiQualifier and QualifierHierarchy

[Michael Ernst]

I discussed this with Mahmood.  (See his progress report this week for some
more insightful comments that were helpful.)  I think it's OK to include
the polymorphic qualifier in the hierarchy.  Either requiring all or none
of the @SubtypeOf qualifiers is OK with me.  (I would incline toward none,
and have them added automatically, but there may be good reasons for going
the other way.)

                    -Mike