Note [EValArgQL]

Data constructor EValArgQL represents an argument that has been
partly-type-checked by Quick Look: the first part of `tcApp` has been
done, but not the second, `finishApp` part.

The constuctor captures all the bits and pieces needed to complete
typechecking.  (An alternative would to to store a function closure,
but that's less concrete.)  See Note [Quick Look at value arguments]
in GHC.Tc.Gen.App