[Checkers] QualifierRoot

[Mahmood Ali]

Greetings,

I am considering removing @QualifierRoot (per Mike's suggestion), but  
there are two outstanding issues.

Mike's recommendation in the TODO.txt is:
> Can't GraphQualifierHierarchy just
> determine this from the fact that exactly one of the qualifiers has no
> @SubtypeOf annotation?  I propose we eliminate it.

The two issues are
__Unqualified roots, e.g. Interned __
The Interning type system consist of one qualifier only: @Interned,  
without its supertype @UnInterned.  To specify this behavior and make  
the proposal work, we need to figure out a way to specify that it is a  
subtype of unqualified.

I propose to create a special annotations in the framework  
@Unqualified to be indicate that a qualifier is a subtype of  
unqualified type.  What do you think?

Alternatives:
1. Have an empty list for @SubtypeOf({ /* empty */ }), downside:  
cannot specify a qualifier being a subtype of unqualified and  
something else

2. Have a non-annotation class, e.g. Void.class; so Interned would be  
@SubtypeOf({Void.class}).  But it it would be nice to make SubtypeOf  
accept class<? extends Annotation>.

__Javari QReadOnly and ReadOnly__
Telmo, can you please confirm that ReadOnly is not a subtype of  
QReadOnly.

To my understanding, ReadOnly is a subtype of QReadOnly only within a  
type argument comparison (e.g. List<@ReadOnly Date> a subtype of  
List<@QReadOnly Date>), but not in any other context.  Can you confirm  
that the following code is invalid in Javari:

   List<@QReadOnly Date> l = ...;
   @ReadOnly Date roDate = ...;
   lst.add(roDate); // invalid

I am about to add a test of this to the Javari test suite.

- Mahmood