This patch addresses #26004.  The root cause was that simplifyInfer
was willing to unify variables "far out".  The fix, in
runTcSWithEvBinds', is to initialise the inert set given-eq level with
the current level. See
    (TGE6) in Note [Tracking Given equalities]
in GHC.Tc.Solver.InertSet

Two loosely related refactors:

* Refactored approximateWCX to return just the free type
  variables of the un-quantified constraints.  That avoids duplication
  of work (these free vars are needed in simplifyInfer) and makes it
  clearer that the constraints themselves are irrelevant.

* A little local refactor of TcSMode, which reduces the number of
  parameters to runTcSWithEvBinds