Executives at WPP Plc, Altice NV and Cap Gemini SA, gathered in Spain for an investor conference this week, voiced confidence that their businesses in the U.S. would profit under President-elect Donald Trump. Clients are more optimistic than they were right after the Nov. 8 election, said Paul Hermelin, chief executive officer of Cap Gemini, the French computer-services firm whose U.S. customers include General Electric Co.