template <class Key, class T, class Hash, class Pred, class Alloc>
void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
unordered_multimap<Key, T, Hash, Pred, Alloc>& y)
noexcept(noexcept(x.swap(y)));
Effects: As if by x.swap(y).