In "A Simple Favor," the stylish new thriller from "Bridesmaids" director Paul Feig, Blake Lively plays a martini-swilling two-faced fashionista. And her swaggering pantsuits, couture dresses and femme fatale vibe borrow heavily from Hollywood icons such as Lauren Bacall, Greta Garbo and . . . Feig himself. "Paul is a total sartorial gem," the film's costume designer,...