[libc++] Patch to support forward_list<Incomplete Type>


libc++'s forward_list already distinguished the sentry node from data node,
so this patch is just trivial: use an external traits to get the begin node type
instead of looking at data node's implementation to find it.

fwdlst_rec.patch (2.78 KB)

Sorry for the long delay.

This looks fine to me; please commit.

— Marshall

I've committed this as r200814.