Fix the instantiation of data constructors in the GHCi debugger

This bug caused Trac #7386, because in the (rather tricky) "type
inference" (aka run time type reconstruction) done by the GHCi
debugger, we were failing to instantiate a data type family
correctly.  When that code was written we didn't *have* data
families.

I wrote Note [Constructor arg types] to explain the new scheme.