ADT: Add ilist_iterator conversions to/from ilist_node

Allow an ilist_iterator to be constructed from an ilist_node, and give
access to the underlying ilist_node as well.

This will be used immediately in lld to support a type-erasure use case.
Longer term, they'll stick around once the iterator is using
ilist_node<NodeTy>* instead of NodeTy*.

llvm-svn: 278467