Investors’ near-insistent demand for even the riskiest corporate debt is powering a Wall Street lending boom, providing aid for struggling companies even as the coronavirus pandemic still drags on the economy.Companies such as hospital operator Community Health Systems Inc. and newspaper publisher Gannett Co. have issued a record $139 billion of bonds and loans with below-investment-grade ratings from the beginning of the year through Feb. 10, according to LCD, a unit of S&P Global Market Intelligence. More than $13 billion of that debt had ratings triple-C or lower-the riskiest tier save for outright default-about twice the previous pace.Despite the onslaught of fresh bonds, riskier companies can now borrow at interest rates once reserved for the most secure type of debt. As of Friday, the average yield for bonds in the ICE BofA US High Yield Index-a group that incorporates embattled retailers and crumbling companies-was just 3.97%. By comparison, the yield on the 10-year US Treasury note, which carries essentially no default risk, was as high as 3.23% less than three years ago. The 10-year Treasury backed down at roughly 1.2% on Friday.