Fix API Annotations for GADT constructors

Authored by alanz on Jun 18 2018, 3:18 AM.

Description

Fix API Annotations for GADT constructors

Summary:
This patch completes the work for Trac #14529 by making sure that all API
Annotations end up attached to a SrcSpan that appears in the final
ParsedSource.

Updates Haddock submodule

Test Plan: ./validate

Reviewers: goldfire, bgamari

Subscribers: rwbarton, thomie, mpickering, carter

GHC Trac Issues: Trac #14529

Differential Revision: https://phabricator.haskell.org/D4867