LOS ANGELES — Then she was the muse to Mick Jagger and company. Now Marianne Faithfull is the inspiration behind a group of young L.A. designers seeking to recapture some of the free-form, slouchy sex appeal of the late Sixties. For fall, they’re showing a blend of urban-bohemian looks — chunky knit sweaters, chic denims, unexpected corduroy suitings and more.