PR libstdc++/90105 make forward_list::sort stable

PR libstdc++/90105 make forward_list::sort stable

While testing the fix I also discovered that operator== assumes the
elements are comparable with operator!= which is not required.

Backport from mainline
2019-04-17  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/90105
	* include/bits/forward_list.tcc (operator==): Do not use operator!= to
	compare elements.
	(forward_list<T, A>::sort(Comp)): When elements are equal take the one
	earlier in the list, so that sort is stable.
	* testsuite/23_containers/forward_list/operations/90105.cc: New test.
	* testsuite/23_containers/forward_list/comparable.cc: Test with
	types that meet the minimum EqualityComparable and LessThanComparable
	requirements. Remove irrelevant comment.

From-SVN: r271010