[analyzer] Let ConstraintManager subclasses provide a more efficient checkNull.

Previously, every call to a ConstraintManager's isNull would do a full
assumeDual to test feasibility. Now, ConstraintManagers can override
checkNull if they have a cheaper way to do the same thing.
RangeConstraintManager can do this in less than half the work.

<rdar://problem/12608209>

llvm-svn: 167138