NEW YORK — The sexual assault charges against Dominique Strauss-Kahn, the head of the International Monetary Fund, have cast uncertainty over global efforts to prevent Europe’s debt crisis from spinning out of control and raise questions about the future of one of the world’s most powerful financial institutions. At the Washington-based IMF, which makes emergency loans to struggling economies, Strauss-Kahn has been a muscular advocate for aiding Greece, Ireland and Portugal as they have fought to avoid insolvency.