Los Angeles city prosecutors are investigating a Costa Mesa-based College Hospital for allegedly taking a mentally ill man 42 miles to the city's skid row and leaving him near the Union Rescue Mission. If the hospital did transport the man from Orange County to Los Angeles County, it might have run afoul of state law that makes it a misdemeanor to dump patients in another county. During the last 2 1/2 years, Los Angeles City Atty. Rocky Delgadillo's office has filed criminal charges against a hospital, sued several other medical facilities and extracted an extensive settlement from Kaiser hospitals for dumping homeless patients.