libstdc++: Make views::single/iota/istream SFINAE-friendly [PR108362]

	PR libstdc++/108362

libstdc++-v3/ChangeLog:

	* include/std/ranges (__detail::__can_single_view): New concept.
	(_Single::operator()): Constrain it.  Move [[nodiscard]] to the
	end of the function declarator.
	(__detail::__can_iota_view): New concept.
	(_Iota::operator()): Constrain it.  Move [[nodiscard]] to the
	end of the function declarator.
	(__detail::__can_istream_view): New concept.
	(_Istream::operator()): Constrain it.  Move [[nodiscard]] to the
	end of the function declarator.
	* testsuite/std/ranges/iota/iota_view.cc (test07): New test.
	* testsuite/std/ranges/istream_view.cc (test08): New test.
	* testsuite/std/ranges/single_view.cc (test07): New test.