Donald J. Trump tried to quash the latest controversy engulfing his campaign, suggesting on Friday that he was not serious when he said that President Obama and Hillary Clinton were the “founders” of the Islamic State terrorist group. After making the suggestion at a rally on Wednesday night, Mr. Trump doubled-down on the assertion on Thursday, insisting in multiple interviews that he really did mean it when he said that the president and Mrs. Clinton were responsible for ISIS. But in an early-morning Twitter post, Mr. Trump said that he was just being sarcastic.