WASHINGTON — The U.S. Supreme Court today upheld the nationwide tax subsidies underpinning President Barack Obama’s health-care overhaul, rejecting a major challenge to the landmark law in a ruling that preserves health insurance for millions of Americans.The justices said in a 6-3 ruling that the subsidies that 8.7 million people currently receive to make insurance affordable do not depend on where they live, as opponents contended.The outcome was the second major victory for Obama in politically charged …