Note [Desugar OpApp in the typechecker]

Operator sections are desugared in the renamer; see GHC.Rename.Expr
Note [Handling overloaded and rebindable constructs].
But for reasons explained there, we rename OpApp to OpApp.  Then,
here in the typechecker, we desugar it to a use of ExpandedThingRn.
That makes it possible to typecheck something like
     e1 `f` e2
where
   f :: forall a. t1 -> forall b. t2 -> t3