Black Flies,” the Sean Penn and Tye Sheridan film about emergency medical first responders, smacked the Cannes Film Festival in the face with a brutal world premiere on Thursday. Splattered brains, dead dogs, an addict giving birth with a needle dangling from her arm — these and a litany of other horrors confronted Penn and Sheridan, who play veteran and rookie paramedics, respectively, at the New York Fire Department. Interestingly enough, the black-tie screening at the Grand Palais enjoyed the dose of reality, giving the film a five-minute standing ovation. “We carry the misery,” a weary Penn tells Sheridan in the film of their chosen profession. That’s an understatement, as chaos unfolds neighborhood by neighborhood in a portrait of an unforgiving city.