[Checkers] @Raw in the nullness type qualifier hierarchy

[Michael Ernst]

Mahmood-

If a reference is raw, then that affects the nullness of its fields, but
not of the reference itself.  Therefore, I don't see that these belong in a
single type hierarchy.  In particular, I could have a reference that is
nullable and raw, or one that is non-null and raw.

This is analogous to how I could have a nullable prototype invariant, or a
non-null prototype invariant.  Whether a reference is a prototype affects
the nullness of its fields, but not of the reference itself.  Thus,
prototype and nullness are separate type hierarchies.  And I think rawness
and nullness should be the same.

                    -Mike