The issue is that in
    instnace C T where
      data S = ...
      f = ...
neither S nor f is really a binder; they are *occurrences*.  Moreover
Haskell dictates that these particular occurrences are disambiguated
by looking at the class whose instance they occur in.

Some of this was not handled right for associated types.  And
RnNames.getLocalNonValBinders was a bit messhy; this patch tidies it
up.

(And thenM is finally gone from RnSource.)