SPECIALISE instance pragmas allow nested `forall`s/contexts

GHC rejects nested foralls and contexts in instance declarations, like so:

class C a

instance forall a. forall b. C (Either a b)

Bug.hs:6:20: error:
    Instance head cannot contain nested ‘forall’s or contexts
    In an instance declaration
  |
6 | instance forall a. forall b. C (Either a b)
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^

However, GHC permits nested foralls in SPECIALISE instance pragmas, like in the following example:

instance C (Either a b) where
  {-# SPECIALISE instance forall a. forall b. C (Either a b) #-}

This seems inconsistent. If we reject nested foralls in the former, we should reject them in the latter as well.