For moviegoers, August is no longer the total disreputable dumping ground it once was. Yet if you want to know whether that phenomenon known as the “August movie” is alive and kicking, look no further than “The Meg.” It’s a big, crass, brainlessly expensive B-movie leftover all dressed up to look like the real deal in blockbuster goods. In other words, it’s a film that has all the August qualities. It takes a cast of flatly “likable” second-tier actors and…