libstdc++: Correctly implement LWG 3946 changes to const_iterator_t [PR122842]

libstdc++: Correctly implement LWG 3946 changes to const_iterator_t [PR122842]

LWG 3946 made const_iterator_t/sentinel_t agree with ranges::cbegin/cend
by defining the aliases in terms of the CPOs, but I defined it the other
way around in an incorrect way that made the aliases not consider
range-ness of const T via __possibly_const_range.  This patch
reimplements the proposed resolution in a more obviously correct way,
mirroring the wording.

	PR libstdc++/122842

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (__access:_CBegin): Define in
	terms of const_iterator directly, not const_iterator_t.
	(__access::_CEnd): Likewise in terms of const_sentinel vs
	const_sentinel_t.
	(const_iterator_t): Move down definition and define in terms
	of ranges::cbegin as per LWG 3946.
	(const_sentinel_t): Likewise in terms of ranges::cend.
	* testsuite/24_iterators/const_iterator/1.cc (test02): Correct
	test for int[], std::array and std::vector.  Also test
	std::string.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>