libstdc++: Conditionalize LWG 3569 changes to join_view

libstdc++: Conditionalize LWG 3569 changes to join_view

LWG 3569 adjusted join_view's iterator specification to handle non
default-constructible iterators by wrapping the corresponding data member
in std::optional, which we followed suit in r13-2649-g7aa80c82ecf3a3.

But this wrapping is unnecessary for iterators that are already
default-constructible.  Rather than unconditionally using std::optional
here, which introduces time/space overhead, this patch conditionalizes
our LWG 3569 changes on the iterator in question being non-forward (and
thus non default-constructible).  We check forwardness instead of
default-constructibility in order to accommodate input-only iterators
that satisfy but do not model default_initializable, e.g. whose default
constructor is underconstrained.

libstdc++-v3/ChangeLog:

	* include/std/ranges (join_view::_Iterator::_M_satisfy):
	Adjust to handle non-std::optional _M_inner as per before LWG 3569.
	(join_view::_Iterator::_M_get_inner): New.
	(join_view::_Iterator::_M_inner): Don't wrap in std::optional if
	the iterator is forward.  Initialize.
	(join_view::_Iterator::operator*): Use _M_get_inner instead
	of *_M_inner.
	(join_view::_Iterator::operator++): Likewise.
	(join_view::_Iterator::iter_move): Likewise.
	(join_view::_Iterator::iter_swap): Likewise.

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