libstdc++: Implement ranges::concat_view from P2542R7

libstdc++: Implement ranges::concat_view from P2542R7

libstdc++-v3/ChangeLog:

	* include/bits/version.def (ranges_concat): Define.
	* include/bits/version.h: Regenerate.
	* include/std/ranges (__detail::__concat_reference_t): Define
	for C++26.
	(__detail::__concat_value_t): Likewise.
	(__detail::__concat_rvalue_reference_t): Likewise.
	(__detail::__concat_indirectly_readable_impl): Likewise.
	(__detail::__concat_indirectly_readable): Likewise.
	(__detail::__concatable): Likewise.
	(__detail::__all_but_last_common): Likewise.
	(__detail::__concat_is_random_access): Likewise.
	(__detail::__concat_is_bidirectional): Likewise.
	(__detail::__last_is_common): Likewise.
	(concat_view): Likewise.
	(__detail::__concat_view_iter_cat): Likewise.
	(concat_view::iterator): Likewise.
	(views::__detail::__can_concat_view): Likewise.
	(views::_Concat, views::concat): Likewise.
	* testsuite/std/ranges/concat/1.cc: New test.

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