Make forall a keyword (#23719)

Before this change, GHC used to accept forall as a term-level identifier:

-- from constraints-0.13
forall :: forall p. (forall a. Dict (p a)) -> Dict (Forall p)
forall d = ...

Now it is a parse error.

The -Wforall-identifier warning has served its purpose and is now a deprecated no-op.