ADT: Clean up docs and formatting for ilist_traits, NFC

This is a prep commit before splitting up ilist_node_traits and
updating/simplifying call sites.
- Move to top of file (I considered moving to a different file,
  llvm/ADT/ilist_traits.h, but it's really not much code).
- Clang-format.
- Convert comments to doxygen, clean them up, and add TODOs for what I'm
  doing next.

llvm-svn: 280109