Remove some more code duplication in std::optional

Hoist the duplicated code from the _Optional_payload partial
specializations into the _Optional_payload_base base class.

	* include/std/optional (_Optional_payload_base::_M_copy_assign): New
	member function to perform non-trivial assignment.
	(_Optional_payload_base::_M_move_assign): Likewise.
	(_Optional_payload<T, true, false, true>::operator=)
	(_Optional_payload<T, true, true, false>::operator=)
	(_Optional_payload<T, true, false, false>::operator=): Call
	_M_copy_assign and/or _M_move_assign to do non-trivial assignments.

From-SVN: r267761