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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@63844 91177308-0d34-0410-b5e6-96231b3b80d8