Fresh off a walking tour of blighted Flint, Mich., on Wednesday, Vivek Ramaswamy spoke excitedly about a comeback for the “forgotten America” that he has made a part of his long-shot bid for the presidency. He wasn’t promising that the automakers that had largely abandoned Flint would return. “We have opportunities, though, to look to the future of a lot that we need to bring to this country,” Mr. Ramaswamy, a 38-year-old entrepreneur, said, ticking through the industries that he’d like to see help drive a revival: semiconductors, pharmaceuticals, defense…