Note [Cloning CoVars and TyVars]

Normally we don't need to clone TyVars and CoVars, but there is one occasion
when we do (see #24463).  When we have
    case unsafeEqualityProof ... of UnsafeRefl g -> ...
we try to float it, using UnsafeEqualityCase.
Why?  See (U3) in Note [Implementing unsafeCoerce]

Alas, floating it widens the scope of `g`, and that led to catastrophe in
#24463, when two identically-named g's shadowed.

Solution: clone `g`; see `cpCloneCoVarBndr`.

BUT once we clone `g` we must apply the cloning substitution to all types
and coercions.  But that in turn means that, given a binder like
   /\ (a :: kind |> g). blah
we must substitute in a's kind, and hence need to substitute for `a`
itself in `blah`.

So our plan is:
  * Maintain a full Subst in `cpe_subst`

  * Clone a CoVar when we we meet an `isUnsafeEqualityCase`;
    otherwise TyVar/CoVar binders are never cloned.

  * So generally the TCvSubst is empty

  * Apply the substitution to type and coercion arguments in Core; but
    happily `substTy` has a no-op short-cut for an empty TCvSubst, so this
    is usually very cheap.

  * In `cpCloneBndr`, for a tyvar/covar binder, check for an empty substitution;
    in that case just do nothing