One in five Chinese LED lighting companies may fail this year as falling prices and oversupply batter an industry that Beijing bankrolled to try to build an energy-efficient future.   About 4,000 companies in China are producing LEDs, or light emitting diodes, tempted by tax breaks, subsidies and offers of cheap land for factories. Now they are locked in intense competition that has halved prices over the past three years.   Just as over-investment and sagging exports dragged down Beijing's solar panel and wind turbine champions, China&#...