Rising stocks and rock-bottom interest rates have delivered a big perk to rich Americans: cheap loans that they can use to fund their lifestyles while minimizing their tax bills. Banks say their wealthy clients are borrowing more than ever before, often using loans backed by their portfolios of stocks and bonds. Morgan Stanley wealth-management clients have $68.1 billion worth of securities-based and other nonmortgage loans outstanding, more than double five years earlier. Bank of America Corp. said it has $62.4 billion in securities-based loans, dwarfing its book of home-equity lines of credit.