Panic with QuantifiedConstraints + RequiredTypeArguments

This code causes a panic in master:

{-# LANGUAGE QuantifiedConstraints, RequiredTypeArguments #-}
module T where

f :: (forall a -> Eq a) => a
f = f

<no location info>: error:
    panic! (the 'impossible' happened)
  GHC version 9.9.20231110:
	instDFunTypes
  df_aIu
  [Just a_aIB[sk:2]]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:186:37 in ghc-9.9-inplace:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/Tc/Utils/Instantiate.hs:371:16 in ghc-9.9-inplace:GHC.Tc.Utils.Instantiate
  CallStack (from HasCallStack):
    panic, called at compiler/GHC/Utils/Error.hs:511:29 in ghc-9.9-inplace:GHC.Utils.Error

I think we should enforce that forall a -> ... is always used with TYPE rep. For forall a. ..., the body can also be a constraint; the situation is more complicated and covered by #22063. Finally, we accept forall {a}. Eq a which seems to give the same result as forall a. Eq a, presumably we should reject it.