Scores of self-described free speech supporters met on Boston Common for a “Resist Marxism” rally aimed at denouncing the perceived efforts of Mayor Martin J. Walsh and black-clad counterprotesters to squelch their constitutional rights, with the promise they’d return in the spring.The rally, a reprise of a similarly tense event in August, drew hundreds of counterprotesters, including a group of anti-fascist or “Antifa” demonstrators.