Add a helper method to parse invariant declarations. This pulls out more functionality from the grammar into simple c++ source files. BUG=angle:711 Change-Id: I19121d710dc1543585361cb53a0deb2ea0479db9 Reviewed-on: https://chromium-review.googlesource.com/212938 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Nicolas Capens <capn@chromium.org>