Warn instead of stopping on misplaced Haddock comments

Given a simple module like

module H where

data F = F () -- ^ Comment for first type argument
           ()

and trying to run Haddock on it, we'll get back

/tmp/H.hs:4:12: parse error on input ‘(’

The error message is rather sub-par in this scenario. It'd be great if we could instead print a warning saying that a Haddock comment is unexpected in that position and then continue on parsing.

Best case scenario would be a more informative message such as “Documenting each constructor argument is not supported” but that might be quite a bit harder.

Filing on GHC Trac as it's the parser that needs changing I believe.

Trac metadata

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