libstdc++: Implement ranges::adjacent_transform_view from P2321R2

libstdc++-v3/ChangeLog:

	* include/std/ranges (__detail::__unarize): Define.
	(adjacent_view::_Iterator): Befriend adjacent_transform_view.
	(adjacent_transform_view): Define.
	(adjacent_transform_view::_Iterator): Define.
	(adjacent_transform_view::_Sentinel): Define.
	(views::__detail::__can_adjacent_transform_view): Define.
	(views::_AdjacentTransform): Define.
	(views::adjacent_transform): Define.
	(views::pairwise_transform): Define.
	* testsuite/std/ranges/adaptors/adjacent_transform/1.cc: New test.