Don't barf on 'HsSpliceTy' (haskell#745)

This handles 'HsSpliceTy's by replacing them with what they expand to.
IIUC everything that is happening, 'renameHsSpliceTy' should not be
able to fail for the inputs we feed it from GHC.

This fixes haskell#574.

(cherry picked from commit 1446450)

haddock-api/src/Haddock/Interface/Rename.hs

@@ -274,10 +274,21 @@ renameType t = case t of
   (XHsType (NHsCoreTy a))   -> pure (XHsType (NHsCoreTy a))
   HsExplicitListTy x i b    -> HsExplicitListTy x i <$> mapM renameLType b
   HsExplicitTupleTy x b     -> HsExplicitTupleTy x <$> mapM renameLType b
-  HsSpliceTy _ _            -> error "renameType: HsSpliceTy"
+  HsSpliceTy _ s            -> renameHsSpliceTy s
   HsWildCardTy a            -> HsWildCardTy <$> renameWildCardInfo a
   HsAppsTy _ _              -> error "renameType: HsAppsTy"
 
+-- | Rename splices, but _only_ those that turn out to be for types.
+-- I think this is actually safe for our possible inputs:
+--
+--  * the input is from after GHC's renamer, so should have an 'HsSpliced'
+--  * the input is typechecked, and only 'HsSplicedTy' should get through that
+--
+renameHsSpliceTy :: HsSplice GhcRn -> RnM (HsType DocNameI)
+renameHsSpliceTy (HsSpliced _ _ (HsSplicedTy t)) = renameType t
+renameHsSpliceTy (HsSpliced _ _ _) = error "renameHsSpliceTy: not an HsSplicedTy"
+renameHsSpliceTy _ = error "renameHsSpliceTy: not an HsSpliced"
+
 renameLHsQTyVars :: LHsQTyVars GhcRn -> RnM (LHsQTyVars DocNameI)
 renameLHsQTyVars (HsQTvs { hsq_explicit = tvs })
   = do { tvs' <- mapM renameLTyVarBndr tvs