Suddenly, Charlotte Gainsbourg is everywhere. Having just stepped out on the Cannes red carpet — in a daringly short, silver-studded Saint Laurent mini, no less — for the premiere of Ismael’s Ghosts (Les fantômes d’Ismaël), the ethereal actress slips into a glittery one-shouldered number, a signature of designer Anthony Vaccarello, for the teaser video as the face of Saint Laurent for fall…