Fix a QuickLook bug

This MR fixes the bug exposed by #24676.  The problem was that
quickLookArg was trying to avoid calling tcInstFun unnecessarily; but
it was in fact necessary.  But that in turn forced me into a
significant refactoring, putting more fields into EValArgQL.

Highlights: see Note [Quick Look overview] in GHC.Tc.Gen.App

* Instantiation variables are now distinguishable from ordinary
  unification variables, by level number = QLInstVar. This is
  treated like "level infinity".  See Note [The QLInstVar TcLevel]
  in GHC.Tc.Utils.TcType.

* In `tcApp`, we don't track the instantiation variables in a set Delta
  any more; instead, we just tell them apart by their level number.

* EValArgQL now much more clearly captures the "half-done" state
  of typechecking an argument, ready for later resumption.
  See Note [Quick Look at value arguments] in GHC.Tc.Gen.App

* Elminated a bogus (never used) fast-path in
  GHC.Tc.Utils.Instantiate.instCallConstraints
  See Note [Possible fast path for equality constraints]

Many other small refactorings.