BOSTON -- If not for the tens of thousands of immigrants who have settled in Massachusetts over the past 15 years, the state's labor force would have shrunk up to 100,000 people and wreaked havoc on the economy, a new study reports. The immigrant population in Massachusetts rose by 35 percent from 1990 to 2000, according to Census data analyzed by the Massachusetts Institute for a New Commonwealth. In Greater Lowell, the growth rate was 38 percent, slightly higher than the state average, thanks in part to a surge in the number of foreign-born residents moving to the suburbs. “People...