LOS ANGELES — The state's largest office building, to be erected downtown, will bear the name of President Ronald Reagan under an executive order signed Monday by Gov. George Deukmejian. Reagan, a former California governor, "has served our nation with great distinction, providing vision and leadership that has again turned America into a nation that is strong, prosperous and committed to opportunity for all residents," Deukmejian said. Ground will be broken soon for the building bounded by 3rd, 4th, Spring and Main streets, with construction expected to be completed by early 1991. The 850,000-square-foot Ronald Reagan Building will have multiple towers, ranging from three to 17 stories, and house 2,300 to 2,500 employees.