Tesla has started production of its electric Model 3 sedan by building souped-up, expensive versions of the car first, with base prices ranging from $49,000 to $64,000. The long-awaited $35,000 edition — the middle-class car CEO Elon Musk has promised for years — will come later. But the Model 3’s buyers clearly don’t mind. Many of them, it turns out, are trading up.