[analyzer] Handle when the dynamic type is worse than the static type.

Currently we don't update the dynamic type of a C++ object when it is
cast. This can cause the situation above, where the static type of the
region is now known to be a subclass of the dynamic type.

Once we start updating DynamicTypeInfo in response to the various kinds
of casts in C++, we can re-add this assert to make sure we don't miss
any cases. This work is tracked by <rdar://problem/12287087>.

In -Asserts builds, we will simply not return any runtime definition
when our DynamicTypeInfo is known to be incorrect like this.

llvm-svn: 163745