With his classic 1990s heartthrob looks, it was only a matter of time before Paul Mescal found his way into a film set in that era. Like “Normal People,” the series that catapulted the young Irish actor to fame, Charlotte Wells’ “Aftersun” has him wearing shorts, but this time with a belt and the polo shirt tucked in — a detail like many others in the film that should send a wave of recognition through audience members who grew up then.