ApiAnnotations : mkGadtDecl discards annotations for HsFunTy

Summary:
When mkGadtDecl is presented wih a HsFunTy it discards the SrcSpan, thus
disconnecting any annotations on the HsFunTy.

```
mkGadtDecl names (L ls (HsForAllTy imp Nothing qvars cxt tau))
  = return $ mk_gadt_con names
  where
    (details, res_ty)           -- See Note [Sorting out the result type]
      = case tau of
          L _ (HsFunTy (L l (HsRecTy flds)) res_ty)
                                            -> (RecCon (L l flds), res_ty)
          _other                                    -> (PrefixCon [], tau)
...
```

This can be triggered by the following

```
{-# LANGUAGE GADTs #-}
module GADTRecords2 (H1(..)) where

-- | h1
data H1 a b where
  C3 :: (Num a) => { field :: a -- ^ hello docs
                   } -> H1 Int Int
```

Test Plan: ./validate

Reviewers: hvr, austin

Reviewed By: austin

Subscribers: bgamari, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D848

GHC Trac Issues: #10309

1 files changed, 2 insertions, 1 deletions

diff --git a/compiler/parser/Parser.y b/compiler/parser/Parser.y
index 1baf606173..3d9b2a39cb 100644
--- a/compiler/parser/Parser.y
+++ b/compiler/parser/Parser.y
@@ -1621,7 +1621,8 @@ type :: { LHsType RdrName }
         : btype                         { $1 }
         | btype qtyconop type           { sLL $1 $> $ mkHsOpTy $1 $2 $3 }
         | btype tyvarop  type           { sLL $1 $> $ mkHsOpTy $1 $2 $3 }
-        | btype '->'     ctype          {% ams (sLL $1 $> $ HsFunTy $1 $3)
+        | btype '->'     ctype          {% ams $1 [mj AnnRarrow $2]
+                                        >> ams (sLL $1 $> $ HsFunTy $1 $3)
                                                [mj AnnRarrow $2] }
         | btype '~'      btype          {% ams (sLL $1 $> $ HsEqTy $1 $3)
                                                [mj AnnTilde $2] }