libstdc++: Fix constraints on std::optional converting assignments [PR117858]

libstdc++: Fix constraints on std::optional converting assignments [PR117858]

It looks like I copied these constraints from operator=(U&&) and didn't
correct them to account for the parameter being optional<U> not U.

libstdc++-v3/ChangeLog:

	PR libstdc++/117858
	* include/std/optional (operator=(const optional<U>&)): Fix copy
	and paste error in constraints.
	(operator=(optional<U>&&)): Likewise.
	* testsuite/20_util/optional/assignment/117858.cc: New test.