ANY lingering doubts that David Letterman detests Bill O'Reilly will be laid to rest tonight, when the gap-toothed funnyman has the conservative Fox News powerhouse on his CBS "Late Show" and machine-guns him with insults. In a tape previewed by Page Six, things go downhill fast as O'Reilly sits down and jokingly presents the liberal-leaning host with a plastic sword to do battle and holds up a plastic shield to defend himself. An irritated Letterman cracks, "Oh, that's nice, that's cute, you come out with toys . . . Am I right about one thing: You guys over there at...