REVERE, Mass. — The Massachusetts manufacturer of such iconic candies as Necco wafers, Clark bars and those heart-shaped Sweethearts decorated with romantic sayings says it will shut down and lay off nearly 400 workers unless it finds a buyer. Necco CEO Michael McGee wrote in a letter last week to Revere Mayor Brian Arrigo the company would close its plant in the city north of Boston if it can’t find a buyer by May 6.