mkGADTVars panic with a GADT data family instance and unused variable in forall
```haskell
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneKindSignatures #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
module T23784 where
import Data.Kind
type Sing :: forall k. k -> Type
data family Sing a
data instance forall a b. Sing a where
S :: Sing '()
```
```
<no location info>: error:
panic! (the 'impossible' happened)
GHC version 9.9.20230724:
mkGADTVars
[k_awT[sk:1], b_awF[sk:1], k_awU[sk:1], a_awE[sk:1]]
<InScope = {a_awE[sk:1] k_awU[sk:1]}
IdSubst = []
TvSubst = [awE :-> '(), awU :-> ()]
CvSubst = []>
Call stack:
CallStack (from HasCallStack):
callStackDoc, called at compiler\GHC\Utils\Panic.hs:191:37 in ghc-9.9-inplace:GHC.Utils.Panic
pprPanic, called at compiler\GHC\Tc\TyCl.hs:4244:9 in ghc-9.9-inplace:GHC.Tc.TyCl
CallStack (from HasCallStack):
panic, called at compiler\GHC\Utils\Error.hs:503:29 in ghc-9.9-inplace:GHC.Utils.Error
Please report this as a GHC bug: https://www.haskell.org/ghc/reportabug
```
I think this is to do with the way we create a representation `TyCon` for a data family instance using the full set of bound variables in the `forall` (see the use of `ty_binders` in `mkAlgTyCon` in `GHC.Tc.TyCl.Instance.tcDataFamInstDecl` and Note [Data type families] in `GHC.Core.TyCon`).
I ran into this when working on error messages for unused type variables in family instances in !11032. I will investigate, but pinging @rae as the authority on this code (in case anything obvious springs to mind).
issue