ADT: Move ilist_node_access to ilist_detail::NodeAccess...

... and make a few ilist-internal API changes, in preparation for
changing how ilist_node is templated.  The only effect for ilist users
should be changing the friend target from llvm::ilist_node_access to
llvm::ilist_detail::NodeAccess (which is only necessary when they
inherit privately from ilist_node).
- Split out SpecificNodeAccess, which has overloads of getNodePtr and
  getValuePtr that are untemplated.
- Use more typedefs to prevent more changes later.
- Force inheritance to use *NodeAccess (to emphasize that ilist *users*
  shouldn't be doing this).

There should be no functionality change here.

llvm-svn: 281142