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")

Test Plan:
This patch was already reviewed and even merged. The patch is later
reverted because apparently it broke the build some time between the
validation of this patch and merge. Creating this new ticket to fix the
validation.

Reviewers: goldfire, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1277

GHC Trac Issues: #10891