Register EpToken in Parser.PostProcess.Haddock (#22558)

This change allows us to reject more badly placed Haddock comments. Examples:

module
  -- | Bad comment for the module
  T17544_kw where

data Foo -- | Bad comment for MkFoo
  where MkFoo :: Foo

newtype Bar -- | Bad comment for MkBar
  where MkBar :: () -> Bar

class Cls a
    -- | Bad comment for clsmethod
  where
    clsmethod :: a

Fixes #22558 (closed)