libstdc++: Implement LWG 3820/3849 changes to cartesian_product_view
The LWG 3820 testcase revealed a bug in _M_advance, which this patch
also fixes.
libstdc++-v3/ChangeLog:
* include/std/ranges
(cartesian_product_view::_Iterator::_Iterator): Remove
constraint on default constructor as per LWG 3849.
(cartesian_product_view::_Iterator::_M_prev): Adjust position
of _Nm > 0 test as per LWG 3820.
(cartesian_product_view::_Iterator::_M_advance): Perform bounds
checking only on sized cartesian products.
* testsuite/std/ranges/cartesian_product/1.cc (test08): New test.