DETROIT — General Motors Corp., anxiously conserving cash so it can keep operating into 2009, said Wednesday it would halt construction of a plant tied to one of its most important projects while the automaker awaits a Washington bailout. GM said it is putting the brakes on the construction of a factory in Flint, Mich., set to make 1.4-liter engines for the Chevrolet Cruze and the Chevy Volt plug-in electric car.