libstdc++: Implement P2278R4 "cbegin should always return a constant iterator"

libstdc++: Implement P2278R4 "cbegin should always return a constant iterator"

This also implements the approved follow-up LWG issues 3765, 3766, 3769,
3770, 3811, 3850, 3853, 3862 and 3872.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (const_iterator_t): Define for C++23.
	(const_sentinel_t): Likewise.
	(range_const_reference_t): Likewise.
	(constant_range): Likewise.
	(__cust_access::__possibly_const_range): Likewise, replacing ...
	(__cust_access::__as_const): ... this.
	(__cust_access::_CBegin::operator()): Redefine for C++23 as per P2278R4.
	(__cust_access::_CEnd::operator()): Likewise.
	(__cust_access::_CRBegin::operator()): Likewise.
	(__cust_access::_CREnd::operator()): Likewise.
	(__cust_access::_CData::operator()): Likewise.
	* include/bits/ranges_util.h (ranges::__detail::__different_from):
	Make it an alias of std::__detail::__different_from.
	(view_interface::cbegin): Define for C++23.
	(view_interface::cend): Likewise.
	* include/bits/stl_iterator.h (__detail::__different_from): Define.
	(iter_const_reference_t): Define for C++23.
	(__detail::__constant_iterator): Likewise.
	(__detail::__is_const_iterator): Likewise.
	(__detail::__not_a_const_iterator): Likewise.
	(__detail::__iter_const_rvalue_reference_t): Likewise.
	(__detail::__basic_const_iter_cat):: Likewise.
	(const_iterator): Likewise.
	(__detail::__const_sentinel): Likewise.
	(const_sentinel): Likewise.
	(basic_const_iterator): Likewise.
	(common_type<basic_const_iterator<_Tp>, _Up>): Likewise.
	(common_type<_Up, basic_const_iterator<_Tp>>): Likewise.
	(common_type<basic_const_iterator<_Tp>, basic_const_iterator<Up>>):
	Likewise.
	(make_const_iterator): Define for C++23.
	(make_const_sentinel): Likewise.
	* include/std/ranges (__cpp_lib_ranges_as_const): Likewise.
	(as_const_view): Likewise.
	(enable_borrowed_range<as_const_view>): Likewise.
	(views::__detail::__is_ref_view): Likewise.
	(views::__detail::__can_is_const_view): Likewise.
	(views::_AsConst, views::as_const): Likewise.
	* include/std/span (span::const_iterator): Likewise.
	(span::const_reverse_iterator): Likewise.
	(span::cbegin): Likewise.
	(span::cend): Likewise.
	(span::crbegin): Likewise.
	(span::crend): Likewise.
	* include/std/version (__cpp_lib_ranges_as_const): Likewise.
	* testsuite/std/ranges/adaptors/join.cc (test06): Adjust to
	behave independently of C++20 vs C++23.
	* testsuite/std/ranges/version_c++23.cc: Verify value of
	__cpp_lib_ranges_as_const macro.
	* testsuite/24_iterators/const_iterator/1.cc: New test.
	* testsuite/std/ranges/adaptors/as_const/1.cc: New test.