[Checkers] @PolyNull and other polymorphic type qualifiers

[Michael Ernst]

I notice that no @PolyNull type qualifier exists.  It should, along with
@PolyInterned.

Then, the case studies need to be re-done (especially the nullness case
studies).

Would this reduce the number of false positives in the case studies?  Or,
were the numbers already computed with the assumption of a PolyNull
qualifier?

                    -Mike