Enhance "Assumption" logic in BasicConstraintManager when reasoning about...

Enhance "Assumption" logic in BasicConstraintManager when reasoning about regions and symbolic regions.  When assuming whether or not a location is non-null, walk up the region hierarchy until we hit a symbolic region (and test it for null).  This may not be the end all solution, as the notion of what a "symbolic region" is really belongs in the specific subclass of StoreManager.

llvm-svn: 57730