Note [Solving CallStack constraints]

See Note [Overview of implicit CallStacks] in GHc.Tc.Types.Evidence.

Suppose f :: HasCallStack => blah.  Then

* Each call to 'f' gives rise to
    [W] s1 :: IP "callStack" CallStack    -- CtOrigin = OccurrenceOf f
  with a CtOrigin that says "OccurrenceOf f".
  Remember that HasCallStack is just shorthand for
    IP "callStack" CallStack
  See Note [Overview of implicit CallStacks] in GHC.Tc.Types.Evidence

* We canonicalise such constraints, in GHC.Tc.Solver.Dict.canDictNC, by
  pushing the call-site info on the stack, and changing the CtOrigin
  to record that has been done.
   Bind:  s1 = pushCallStack <site-info> s2
   [W] s2 :: IP "callStack" CallStack   -- CtOrigin = IPOccOrigin

* Then, and only then, we can solve the constraint from an enclosing
  Given.

So we must be careful /not/ to solve 's1' from the Givens.  We guarantee
this by canonicalising before looking up in the inert set.