The number of millionaires in the U.S. jumped during the second quarter of this year, according to Fidelity Investments. Thanks to the recent stock market rebound, 378,000 Fidelity investors with 401(k) accounts became millionaires during that three-month period. That is a jump of 10% from the first quarter, when 340,000 achieved that lofty status. Meanwhile, 349,104 Fidelity investors with IRAs…