[analyzer] Don't strip CXXBaseObjectRegions when checking dynamic_casts.

...and /do/ strip CXXBaseObjectRegions when casting to a virtual base class.

This allows us to enforce the invariant that a CXXBaseObjectRegion can always
provide an offset for its base region if its base region has a known class
type, by only allowing virtual bases and direct non-virtual bases to form
CXXBaseObjectRegions.

This does mean some slight problems for our modeling of dynamic_cast, which
needs to be resolved by finding a path from the current region to the class
we're trying to cast to.

llvm-svn: 161797