Previously field names that were listed in the first declarator list
inside a struct declaration were not checked against each other.

BUG=angleproject:2204
TEST=angle_unittests

Change-Id: Ibf821d45556f6dfe0223dae673644f6795daf4cb
Reviewed-on: https://chromium-review.googlesource.com/739825Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>