swift234

Remove all uses of ilist_node::getNextNode() and ilist_node::getPrevNode() in favor of just using iterators.

This change is needed for the next update to ToT LLVM. It can be put
into place now without breaking anything so I am committing it now.

The churn upstream on ilist_node is neccessary to remove undefined
behavior. Rather than updating the different ilist_node patches for the
hacky change required to not use iterators, just use iterators and keep
everything as ilist_nodes. Upstream they want to eventually do this, so
it makes sense for us to just do it now.

Please do not introduce new invocations of
ilist_node::get{Next,Prev}Node() into the tree.