libstdc++: Improve optional's <=> constraint recursion workaround [PR104606]

libstdc++: Improve optional's <=> constraint recursion workaround [PR104606]

It turns out the reason the behavior of this testcase changed after CWG
2369 is because validity of the substituted return type is now checked
later, after constraints.  So a more reliable workaround for this issue
is to add a constraint to check the validity of the return type earlier,
matching the pre-CWG 2369 semantics.

	PR libstdc++/104606

libstdc++-v3/ChangeLog:

	* include/std/optional (operator<=>): Revert r14-9771 change.
	Add constraint checking the validity of the return type
	compare_three_way_result_t before the three_way_comparable_with
	constraint.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>