As reported in Trac #10891, Template Haskell's `reify` was not generating Decls
for associated types. This patch fixes that.

Note that even though `reifyTyCon` function used in this patch returns some
type instances, I'm ignoring that.

Here's an example of how associated types are encoded with this patch:

(Simplified representation)

    class C a where
      type F a :: *

    -->

    OpenTypeFamilyD "F" ["a"]

With default type instances:

    class C a where
      type F a :: *
      type F a = a

    -->

    OpenTypeFamilyD "F" ["a"]
    TySynInstD "F" (TySynEqn [VarT "a"] "a")

Reviewed By: goldfire

Differential Revision: https://phabricator.haskell.org/D1254

GHC Trac Issues: #10891