Machinists and the policy of Section 8(c) “‘to encourage free debate on issues dividing labor and management,’ Brown, 554 U.S. at 67 (quoting Linn v. United Plant Guard Workers, Local 114, 383 U.S. 53, 62 (1966)), employers remain free under this rule—as they have in the past—to express noncoercive views regarding the exerci