Anchorage Capital Group, one of the biggest hedge-fund investors in distressed debt, is closing its flagship fund after 18 years and returning the $7.4 billion it manages to clients, citing a market environment in which cheap money has helped keep stock and bond prices elevated while suppressing corporate defaults. The credit fund, Anchorage Capital Partners, is suspending clients’ ability to get their money back as it is wound down, the New York firm told clients in a letter Wednesday that was viewed by The Wall Street Journal.