WASHINGTON — A day after Secretary of State Rex Tillerson suggested a possible diplomatic breakthrough with nuclear-armed North Korea, President Trump undercut his top diplomat Sunday, saying Tillerson “is wasting his time.” The stunning rebuke was the latest incident in which Trump has publicly contradicted Tillerson, and quickly dashed any hope for progress in easing a perilously volatile dispute with the government in Pyongyang, which has threatened to destroy the United States just as Trump has threatened the same against North Korea. Tillerson said Saturday in Beijing that the U.S.