[opaqueness] fix a bug with disappearing type variables in cloning
[opaqueness] fix a bug with disappearing type variables in cloning

ssaavedra/why3,jpdeplaix/why3,douglas-larocca/why3,ssaavedra/why3,douglas-larocca/why3,jpdeplaix/why3,douglas-larocca/why3,jpdeplaix/why3,ssaavedra/why3