For decades, many of the nation’s biggest companies staked their futures far from the fraying downtowns of aging East Coast and Midwestern cities. One after another, they decamped for sprawling campuses in the suburbs and exurbs. Now, corporate America is moving in the other direction. In June, McDonald’s joined a long list of companies that are returning to downtown Chicago from suburbs like Oak Brook, Northfield and Schaumburg. Later this month, the top executive team at General Electric — whose 70-acre wooded campus in Fairfield, Conn., has embodied the quintessential suburban corporate office park since it opened in 1974 — will move to downtown Boston.