[Checkers] AnnotatedTypeMirror is not sufficiently unique

[Michael Ernst]

Artemus-

As Mahmood (and the Javadoc) noted, AnnotatedTypeMirror represents a type.
In this respect it is like TypeMirror, which it directly mimics.

I agree that it doesn't seem to make sense to have a getElement method in
AnnotatedTypeMirror.

                    -Mike