[Checkers] @Raw in the nullness type qualifier hierarchy

[Michael Ernst]

Mahmood-

> 3. Raw: the type could represent a not-null reference whose fields are  
> not fully initalized.  This is usually the case during the execution  
> of a constructor.

I see that this is a consistent interpretation, but it is not documented
anywhere.  It is surprising to me and might be to other users as well.  I
think it is easier to think about two distinct, simple, and orthogonal
concepts -- nullness and rawness -- than to have only 3 of the 4 logical
combinations, where raw implicitly implies non-null.

My main concern is with conceptual simplicity.  So far, the concrete
difficulty I encountered was with my surprise/confusion, not with
expressiveness.  "this" is never nullable, so a nullable raw reference
would not appear in a constructor, but raw references can propagate to
elsewhere that might admit null references.

I see that combining into a single type hierarchy is an implementation
convenience, but the interface to users should be more important (however
we decide it).

                    -Mike