libstdc++: Implement P2325 changes to default-constructibility of views

libstdc++: Implement P2325 changes to default-constructibility of views

NB: This backport of r12-1606 to the 10 branch deliberately omits parts
of P2325R3 so as to maximize backward compatibility with pre-P2325R3 code.
In particular, we don't remove the default ctors for back_insert_iterator,
front_insert_iterator, ostream_iterator, ref_view and basic_istream_view.
And in the 10 branch we we don't have __non_propagating_cache or the
partial specialization of __box, so the changes to them are omitted too.
Finally, we don't update __cpp_lib_ranges to 202106L because many
significant 202106 Ranges changes still aren't implemented in the 10
branch, e.g. P2281R1 and P2210R2.

===

This implements the wording changes of P2325R3 "Views should not be
required to be default constructible".  Changes are relatively
straightforward, besides perhaps those to __box (which now stands
for copyable-box instead of semiregular-box) and __non_propagating_cache.

For __box, this patch implements the recommended practice to also avoid
std::optional when the boxed type is nothrow_move/copy_constructible.

For __non_propagating_cache, now that it's used by split_view::_M_current,
we need to add assignment from a value of the underlying type to the
subset of the std::optional API implemented for the cache (needed by
split_view::begin()).  Hence the new __non_propagating_cache::operator=
overload.

In passing, this fixes the undesirable list-init in the constructors of
the partial specialization of __box as reported in PR100475 comment #7.

	PR libstdc++/103904

libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h (weakly_incrementable): Remove
	default_initializable requirement.
	* include/bits/stl_iterator.h (common_iterator): Constrain the
	default ctor.
	(counted_iterator): Likewise.
	* include/std/ranges (ranges::view): Remove default_initializable
	requirement.
	(subrange): Constrain the default ctor.
	(__detail::__box::operator=): Handle self-assignment.
	(single_view): Constraint the default ctor.
	(iota_view): Relax semiregular constraint to copyable.
	Constrain the default ctor.
	(iota_view::_Iterator): Constraint the default ctor.
	(filter_view): Likewise.
	(filter_view::_Iterator): Likewise.
	(transform_view): Likewise.
	(transform_view::_Iterator): Likewise.
	(take_view): Likewise.
	(take_view::_Iterator): Likewise.
	(take_while_view): Likewise.
	(take_while_view::_Iterator): Likewise.
	(drop_while_view): Likewise.
	(drop_while_view::_Iterator): Likewise.
	(join_view): Likewise.
	(split_view): Constrain the default ctor.
	(common_view): Likewise.
	(reverse_view): Likewise.
	(elements_view): Likewise.
	(elements_view::_Iterator): Likewise.
	* include/std/span (enable_view<span<_ElementType, _Extent>>):
	Define this partial specialization to true unconditionally.
	* testsuite/std/ranges/p2325.cc: New test.
	* testsuite/std/ranges/single_view.cc (test06): New test.
	* testsuite/std/ranges/view.cc: Adjust now that view doesn't
	require default_initializable.

(cherry picked from commit 4b4f5666b4c2f3aab2a9f3d53d394e390b9b682d)