libstdc++: Implement P2415R2 changes to viewable_range / views::all

libstdc++: Implement P2415R2 changes to viewable_range / views::all

This implements the wording changes in P2415R2 "What is a view?", which
is a DR for C++20.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (__detail::__is_initializer_list):
	Define.
	(viewable_range): Adjust as per P2415R2.
	* include/bits/ranges_cmp.h (__cpp_lib_ranges): Adjust value.
	* include/std/ranges (owning_view): Define as per P2415R2.
	(enable_borrowed_range<owning_view>): Likewise.
	(views::__detail::__can_subrange): Replace with ...
	(views::__detail::__can_owning_view): ... this.
	(views::_All::_S_noexcept): Sync with operator().
	(views::_All::operator()): Use owning_view instead of subrange
	as per P2415R2.
	* include/std/version (__cpp_lib_ranges): Adjust value.
	* testsuite/std/ranges/adaptors/all.cc (test06): Adjust now that
	views::all uses owning_view instead of subrange.
	(test08): New test.
	* testsuite/std/ranges/adaptors/lazy_split.cc (test09): Adjust
	now that rvalue non-view non-borrowed ranges are viewable.
	* testsuite/std/ranges/adaptors/split.cc (test06): Likewise.