CHICAGO--(BUSINESS WIRE)--While optimism for the nation’s economic outlook among U.S. business leaders rose in second quarter 2015, optimism among Greek business leaders plummeted to its lowest level since fourth quarter 2012. This is according to the latest data from the Grant Thornton International Business Report (IBR), a survey of more than 2,500 business leaders in 36 countries. In second quarter 2015, optimism for the economic outlook among U.S. business leaders rose 11 percentage points