ADT: Remove the ilist_nextprev_traits customization point

No one is using the capability to implement next and prev another way
(since lld stopped doing it in r278468).  Remove the customization point
by moving the API from ilist_nextprev_traits<T> to ilist_node_access.

The old traits class is still useful/necessary API as a target for
friends of node types that inherit privately from ilist_node.
Eventually I plan to either remove it entirely or move the template
parameters to the methods.

(Note: if there's desire to bring back customization of next/prev
pointers in the future (e.g., to pack some bits in there), I think a
traits class like this is an awkward way to accomplish it.  Instead, we
should change ilist<T> to be ilist<ilist_node<T>>, and give an extra
template parameter to ilist_node.)

llvm-svn: 278532