libstdc++: elements_view's sentinel and iterator not comparable [LWG 3406]

libstdc++: elements_view's sentinel and iterator not comparable [LWG 3406]

This implements the proposed resolution for LWG 3406, and adds a
testcase for the example from P1994R1.

libstdc++-v3/ChangeLog:

	* include/std/ranges (elements_view::begin): Adjust constraints.
	(elements_view::end): Likewise.
	(elements_view::_Sentinel::operator==): Templatize to take both
	_Iterator<true> and _Iterator<false>.
	(elements_view::_Sentinel::operator-): Likewise.
	* testsuite/std/ranges/adaptors/elements.cc: Add testcase for
	the example from P1994R1.
	* testsuite/std/ranges/adaptors/lwg3406.cc: New test.