By Andrew Chung WASHINGTON (Reuters) -The U.S. Supreme Court on Tuesday declined to block Senator Lindsey Graham from having to testify before a grand jury in a criminal investigation of Donald Trump’s efforts to overturn his 2020 election loss in the state of Georgia, handing a setback to a prominent ally of the former president. The justices denied Graham’s emergency request to put on hold a judge’s order requiring him to appear as a witness before the grand jury in Fulton County while the Republican senator’s appeal in the dispute proceeded. Graham, who represents South Carolina in the Sena…