Zejun Wu authored Sep 20, 2018 and Ben Gamari committed Sep 20, 2018

Encouter following error when `make`:

```
Extension error:
The 'ghc-flag' directive is already registered to domain std
```

as we register `ghc-flag` to `std` in `add_object_type` first and then
overtride it in `add_directive_to_domain`.

Test Plan:
  make -C utils/haddock/doc html SPHINX_BUILD=/usr/bin/sphinx-build

Reviewers: austin, bgamari, patrickdoc

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5161

(cherry picked from commit 8c7d33a8)