A weaker dollar is helping two U.S. food giants, Kellogg Co. and Mondelez International Inc., break out of a prolonged sales slump. The news signals that U.S. food producers are finally getting some relief overseas, where the strong dollar had battered results. That’s helping offset a domestic market that remains sluggish for “big food.” As Americans change the way they eat and shop, many are turning away from traditional brands.