Revert "ADT: Require explicit ilist iterator/pointer conversions"

This reverts commit r252372.  Apparently I missed clang-tools-extra.
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-ubuntu-fast/builds/2534/steps/build/logs/stdio

llvm-svn: 252373