libstdc++: Implement LWG 4027 change to possibly-const-range [PR118083]

libstdc++: Implement LWG 4027 change to possibly-const-range [PR118083]

LWG 4027 effectively makes the const range access CPOs ranges::cfoo behave
more consistently across C++23 and C++20 (pre-P2278R4) and also more
consistently with the std::cfoo range accessors, as the below testcase
adjustments demonstrate (which mostly consist of reverting workarounds
added by r14-3771-gf12e26f3496275 and r13-7186-g0d94c6df183375).

In passing fix PR118083 which reports that the input_range constraint on
possibly-const-range is missing in our implementation.  A consequence of
this is that the const range access CPOs now consistently reject a non-range
argument, and so in some our of tests we need to introduce otherwise
unused begin/end members.

	PR libstdc++/118083

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h
	(ranges::__access::__possibly_const_range): Adjust logic as per
	LWG 4027.  Add missing input_range constraint.
	* testsuite/std/ranges/access/cbegin.cc (test05): Verify LWG
	4027 testcases.
	* testsuite/std/ranges/access/cdata.cc: Adjust, simplify and
	consolidate some tests after the above.
	* testsuite/std/ranges/access/cend.cc: Likewise.
	* testsuite/std/ranges/access/crbegin.cc: Likewise.
	* testsuite/std/ranges/access/crend.cc: Likewise.
	* testsuite/std/ranges/adaptors/join.cc: Likewise.
	* testsuite/std/ranges/adaptors/take_while.cc: Likewise.
	* testsuite/std/ranges/adaptors/transform.cc: Likewise.

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