Back in May, Charli XCX released “Rock Music,” the big-statement lead single from her new album Music, Fashion, Film.Upon arrival, the track felt like a giddy, provocative goof: “I think the dancefloor is dead, so now we’re making rawwwwwwwk music.” Madonna, on the brink of releasing her own dancefloor-centric album, briefly took exception, but…...