libstdc++: Fix elements_view::operator* and operator[] [LWG 3502]

While we're modifying elements_view, this also implements the one-line
resolution of LWG 3492.

libstdc++-v3/ChangeLog:

	* include/std/ranges (__detail::__returnable_element): New
	concept.
	(elements_view): Use this concept in its constraints.  Add
	missing private access specifier.
	(elements_view::_S_get_element): Define as per LWG 3502.
	(elements_view::operator*, elements_view::operator[]): Use
	_S_get_element.
	(elements_view::operator++): Remove unnecessary constraint
	as per LWG 3492.
	* testsuite/std/ranges/adaptors/elements.cc (test05): New test.