[analyzer] Support modeling no-op BaseToDerived casts in ExprEngine.

Introduce a new MemRegion sub-class, CXXDerivedObjectRegion, which is
the opposite of CXXBaseObjectRegion, to represent such casts. Such region is
a bit weird because it is by design bigger than its super-region.
But it's not harmful when it is put on top of a SymbolicRegion
that has unknown extent anyway.

Offset computation for CXXDerivedObjectRegion and proper modeling of casts
still remains to be implemented.

Differential Revision: https://reviews.llvm.org/D51191

llvm-svn: 340984