libstdc++: Reverse arguments in constraint for std::optional's <=> [PR104606]

This is a workaround for a possible compiler bug that causes constraint
recursion in the operator<=>(const optional<T>&, const U&) overload.

libstdc++-v3/ChangeLog:

	PR libstdc++/104606
	* include/std/optional (operator<=>(const optional<T>&, const U&)):
	Reverse order of three_way_comparable_with template arguments.
	* testsuite/20_util/optional/relops/104606.cc: New test.