ADT: Use typedefs for ilist_base and ilist_node_base, NFC

This is a prep commit to minimize changes in a follow-up that is adding
a template parameter to ilist_node_base and ilist_base.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281141 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed