This patch adds support for explicitly-written quantification in
typeclass instances, such as:

  instance forall k (a :: k). C a
  deriving instance forall k (a :: k). D a

It does so by adding a field of type `Maybe (TyVarBndr ())`
to both the `InstanceD` and `StandaloneDerivD` constructors of the
Template Haskell `Dec` datatype, and making appropriate use of it to
ensure that spliced declarations don't silently drop the user-written
quantification.

Fixes #21794

Updates haddock submodule