Erich von Stroheim, William Holden, Gloria Swanson,“Sunset Boulevard” (1950).  Preview audiences were left stunned, oddly amused and utterly confused At the start of “Sunset Boulevard,” hapless screenwriter Joe Gillis (William Holden) floats face-down in a swimming pool with several bullet holes punched into his torso. He’d been the long-term houseguest of deranged former silent screen…