Twenty years ago, millions of Americans were cocking their ears — waiting to hear a "giant sucking sound."They feared Mexico would begin vacuuming up U.S. manufacturing jobs as soon as President Clinton signed the North American Free Trade Agreement, or NAFTA, on Dec. 8, 1993. The "sucking sound" famously had been forecast in 1992 by Ross Perot, the businessman turned independent presidential candidate.Clinton won the election, and began pushing for NAFTA's passage. He promised the sprawling trade agreement would become a wealth generator as it pulled together the economies of Canada, the United States and Mexico.Who turned out to be...