Add a constexpr constructor for TVariable BUG=angleproject:2267 TEST=angle_unittests Change-Id: I9b7e01801caa7235ac5e2d4212ea92e38c1f774d Reviewed-on: https://chromium-review.googlesource.com/908752 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>