libstdc++: Enable three-way comparison for iota_view iterators

libstdc++: Enable three-way comparison for iota_view iterators

The declaration of operator<=> was disabled due to a typo in the macro.
The declaration was also ill-formed when three_way_comparable<_Winc> is
not satisfied, which is a defect in the C++20 draft.

	* include/std/ranges (iota_view::_Iterator): Fix typo in name of
	__cpp_lib_three_way_comparison macro and use deduced return type for
	operator<=>.
	* testsuite/std/ranges/iota/iterator.cc: New test.