Note [Internal used_names]

Most of the used_names are External Names, but we can have System
Names too. Two examples:

* Names arising from Language.Haskell.TH.newName.
  See Note [Binders in Template Haskell] in GHC.ThToHs (and #5362).
* The names of auxiliary bindings in derived instances.
  See Note [Auxiliary binders] in GHC.Tc.Deriv.Generate.

Such Names are always for locally-defined things, for which we don't gather
usage info, so we can just ignore them in ent_map. Moreover, they are always
System Names, hence the assert, just as a double check.