ADT: Remove ilist_*sentinel_traits, NFC

Remove all the dead code around ilist_*sentinel_traits.  This is a
follow-up to gutting them as part of r279314 (originally r278974),
staged to prevent broken builds in sub-projects.

Uses were removed from clang in r279457 and lld in r279458.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279473 91177308-0d34-0410-b5e6-96231b3b80d8
18 files changed