The recent comedies are even less successful than the new musicals. Buddy Buddy is a flat disappointment from veteran filmmaker Billy Wilder, working in tandem with his longtime collaborator, I. A. L. Diamond. Taking their cue from a darkly comic French farce known by more than one title-- ''A Pain in the Neck'' is one of them-- they have cast Jack Lemmon as a would-be suicide who wants to do away with himself in a hotel room and Walter Matthau as a hit man who has an assassination to pull off from an adjoining window. They get in each other's way for the duration of the film, which is more frantic than it is funny. There are some distasteful scenes in a sex clinic.