LONDON — Secretary of State Rex Tillerson has delivered the toughest condemnation yet from a Trump administration official of the persecution of Myanmar’s Rohingya Muslims, likening the violence against them to ethnic cleansing and demanding it stop. On Thursday, Tillerson directed the blame toward Myanmar’s powerful military, which is responsible for security operations that have seen nearly 400,000 people flee to neighboring Bangladesh in the past three weeks after Rohingya insurgents launched coordinated attacks on government forces.