‘Onslaught’ Review: Adria Arjona and Dan Stevens Star in Adam Wingard’s Fun but Scattered Military Slasher

A 12-hour clock counts down to an inevitable massacre in “Onslaught,” the kind of elevated, razor-wire genre romp that director Adam Wingard and writer Simon Barrett used to make before studio outings like “Godzilla vs. Kong.” It isn’t the most polished of the duo’s output, and often fee…