[ifs] Prepare llvm-ifs for elfabi/ifs merging. This diff changes llvm-ifs to use unified IFS file format and perform other renaming changes in preparation for the merging between elfabi/ifs. Differential Revision: https://reviews.llvm.org/D99810 GitOrigin-RevId: 61fa9afe4c5b014181ce41a35c84fb0f3be975db