libstdc++: Implement LWG 3746 for std::optional

This avoids constraint recursion in operator<=> for std::optional.
The resolution was approved in Kona 2022.

libstdc++-v3/ChangeLog:

	* include/std/optional (__is_derived_from_optional): New
	concept.
	(operator<=>): Use __is_derived_from_optional.
	* testsuite/20_util/optional/relops/lwg3746.cc: New test.