WASHINGTON — The International Monetary Fund on Monday raised its forecast for the U.S. economy over the next two years, saying President-elect Donald Trump’s policies should boost economic growth, particularly in 2018. But officials warned that if Trump’s protectionist trade proposals set off a trade war, that could be “quite destructive” for the global economy. The IMF also increased 2017 growth projections for a number of other countries including China, Germany, Japan and Britain, but warned that the global economy faced a number of downside risks from rising protectionism to a jump in interest rates. The 189-nation global lending agency’s latest economic outlook took note of the significant impact Trump’s election has already had in giving a boost to U.S.