libstdc++: Implement P1739R4 changes to views::take/drop/counted

libstdc++: Implement P1739R4 changes to views::take/drop/counted

This implements P1739R4 along with the resolution for LWG 3407 which
corrects the paper's wording.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_util.h (views::_Drop): Forward declare.
	(subrange): Befriend views::_Drop.
	(subrange::_S_store_size): Declare constexpr instead of just
	const, remove obsolete comment.
	* include/std/ranges (views::__detail::__is_empty_view): Define.
	(views::__detail::__is_basic_string_view): Likewise.
	(views::__detail::__is_subrange): Likewise.
	(views::__detail::__is_iota_view): Likewise.
	(views::__detail::__can_take_view): Rename template parm _Tp to _Dp.
	(views::_Take): Rename template parm _Tp to _Dp, make it non-deducible
	and fix it to range_difference_t<_Range>.  Implement P1739R4 and
	LWG 3407 changes.
	(views::__detail::__can_drop_view): Rename template parm _Tp to _Dp.
	(views::_Drop): As with views::_Take.
	(views::_Counted): Implement P1739R4 changes.
	* include/std/span (__detail::__is_std_span): Rename to ...
	(__detail::__is_span): ... this and turn it into a variable
	template.
	(__detail::__is_std_array): Turn it into a variable template.
	(span::span): Adjust uses of __is_std_span and __is_std_array
	accordingly.
	* testsuite/std/ranges/adaptors/p1739.cc: New test.