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

[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>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@167138 91177308-0d34-0410-b5e6-96231b3b80d8