[nomaster] SI-8764 fix return type of case class productElement under Xexperimental

Under Xexperimental, productElement now returns the lub instead
of the weak lub of case class parameter types (numeric widening
shouldn't magically happen *inside* productElement).

This was removed from 2.12.x in 6317ae2.
Merge pull request #3938 from gourlaysama/wip/t8764

SI-8764 fix return type of case class productElement under Xexperimental