WASHINGTON — Donald Trump says he will force Mexico to pay for a border wall as president by threatening to cut off the flow of billions of dollars in payments that immigrants send home, an idea that could decimate the Mexican economy and set up an unprecedented showdown between the United States and a key diplomatic ally.In a two-page memo to the Washington Post, Trump outlined for the first time how he would seek to force Mexico to pay for a 1,000-mile border wall, which he has made a cornerstone of his presidential campaign.The proposal would jeopardize a stream of cash many economists say is vital for Mexico’s struggling economy.