Function::GetDynamicInvocationForwarder creates duplicate entries in the dispatcher cache

[mraleph]

Some functions don't need dynamic invocation forwarder. For such functions we just use the function itself as the forwarder, however we still add it to the cache:

  result = needs_dyn_forwarder ? CreateDynamicInvocationForwarder(mangled_name)
                               : ptr();
  owner.AddInvocationDispatcher(mangled_name, Array::null_array(), result);

This newly added entry will never be found by GetInvocationDispatcher though - because we are looking for functions with a specific kind: kind() == kDynamicInvocationForwarder.