trans: Collector misses closure drop-glue generated via fn-once-shims

[michaelwoerister]

FnOnce shims can introduce a drop of ClosureType, but the translation item collector only ever gets to see the &ClosureType which does not require drop-glue. This leads to errors because the desired drop-glue doesn't get instantiated.

@arielb1 Do I remember correctly that you were looking into FnOnce shims at the MIR level? That might help here.