The typechecker has the invariant that every type should be well-kinded
as it stands, without zonking.  See Note [The well-kinded type invariant]
in TcType.

That invariant was not being upheld, which led to Trac #14174.  I fixed
part of it, but T14174a showed that there was more.  This patch finishes
the job.

* See Note [The tcType invariant] in TcHsType, which articulates an
  invariant that was very nearly, but not quite, true.  One place that
  falisified it was the HsWildCardTy case of tc_hs_type, so I fixed that.

* mkNakedCastTy now makes no attempt to eliminate casts; indeed it cannot
  lest it break Note [The well-kinded type invariant].  The prior comment
  suggested that it was crucial for performance but happily it seems not
  to be. The extra Refls are eliminated by the zonker.

* I found I could tidy up TcHsType.instantiateTyN and instantiateTyUntilN
  by eliminating one of its parameters.  That led to a cascade of minor
  improvements in TcTyClsDecls. Hooray.