This October, while in Berlin to shoot the video for ''Night and Day,'' U2's contribution to the Red Hot + Blue AIDS benefit album, the Irish rockers recorded demos for their own LP, due early next summer. Though Daniel Lanois, who produced the band's '87 chart-topper The Joshua Tree, was in on the sessions, a source says the album will be much more dance-oriented than previous efforts. Clue No. 2: U2 have been dancing at Dublin's Wah Wah Club, where the deejays favor funk and disco. On Halloween, lead singer Bono and drummer Larry Mullen Jr. celebrated Mullen's birthday at a Wah Wah nostalgia party. The theme? Glam rock, circa 1974.