libstdc++: Implement P2540R1 change to views::cartesian_product()
This paper changes the identity element of views::cartesian_product to a
singleton range instead of an empty range. It was approved alongside
the main cartesian_product paper P2374R4, but unfortunately was overlooked
when implementing the main paper.
libstdc++-v3/ChangeLog:
* include/std/ranges (views::_CartesianProduct::operator()):
Adjust identity case as per P2540R1.
* testsuite/std/ranges/cartesian_product/1.cc (test01):
Adjust expected result of the identity case.
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>