Allow -- ^ Haddock syntax on record constructors

Original ticket is on Haddock Trac here which I'm opening here as it requires lexer/parser changes.

The idea is that given something like

-- | A tree with labeled internal nodes
data Tree =
    Empty                -- ^ Leaf
  | Branch               -- ^ Internal node
    { label :: a         -- ^ Node label
    , subtrees :: [Tree] -- ^ List of subtrees
    }

we'd like it to parse as the user would expect instead of failing. We're currently forced to do something like

-- | A tree with labeled internal nodes
data Tree =
    Empty                -- ^ Leaf
  | -- | Internal node
    Branch
    { label :: a         -- ^ Node label
    , subtrees :: [Tree] -- ^ List of subtrees
    }

which is quite ugly. I can't think of a reason why the first form wouldn't be allowed.

Trac metadata

Trac field	Value
Version	7.9
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
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