libstdc++: Fix up implementation of LWG 3533 [PR101589]
In r12-569 I accidentally applied the LWG 3533 change to
elements_view::iterator::base instead to elements_view::base.
This patch corrects this, and also applies the corresponding LWG 3533
change to lazy_split_view::inner-iter::base now that we implement P2210.
PR libstdc++/101589
libstdc++-v3/ChangeLog:
* include/std/ranges (lazy_split_view::_InnerIter::base): Make
the const& overload unconstrained and return a const reference
as per LWG 3533. Make unconditionally noexcept.
(elements_view::base): Revert accidental r12-569 change.
(elements_view::_Iterator::base): Make the const& overload
unconstrained and return a const reference as per LWG 3533.
Make unconditionally noexcept.