Apple is gearing up to build another corporate campus in a yet-to-be-announced American city and increase its workforce across the U.S. by 20,000 new employees over the next five years, the iPhone maker said Wednesday. The news comes packaged with Apple’s announcement that it plans to "invest," or generate, more than $350 billion for the U.S. economy between now and the end of 2023. The Cupertino-based company did not detail how it came up with that dollar figure. Apple says the economic benefit…