[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
68 files changed