Make SymbolicRegion subclass TypedRegion, for symbols usually have types, so

do the symblic regions associated with them and we need them to be typed.

Current SymbolicRegion::getRValueType() method is very restricting. It may be
modified when we are more clear about what could be the types of symblic
regions.

BasicConstraintManager::Assume() is changed due to that now SymblicRegion is a
subclass of SubRegion.
 

llvm-svn: 63844