As a first step towards fixing PR9641, add a CK_DynamicToNull cast kind which

represents a dynamic cast where we know that the result is always null.

For example:

struct A {
  virtual ~A();
};
struct B final : A { };
struct C { };

bool f(B* b) {
  return dynamic_cast<C*>(b);
}

llvm-svn: 129256