libstdc++: Implement P1994R1 changes to ranges::elements_view

libstdc++: Implement P1994R1 changes to ranges::elements_view

The example from the paper doesn't compile without the proposed
resolution for LWG 3406, so we'll add a testcase for this once the
proposed resolution is implemented.

libstdc++-v3/ChangeLog:

	* include/std/ranges (elements_view::end): Replace these two
	overloads with four new overloads.
	(elements_view::_Iterator::operator==): Remove.
	(elements_view::_Iterator::operator-): Likewise.
	(elements_view::_Sentinel): Define.