Qualify stored constant union data with const This prevents accidentally changing data that may be shared between multiple TIntermConstantUnion nodes. Besides making the code less prone to bugs in general, this will make it easier to implement constant folding of array constructors. BUG=541551 TEST=angle_unittests, WebGL conformance tests Change-Id: I4f3059f70b841d9dd0cf20fea4d37684da9cd47e Reviewed-on: https://chromium-review.googlesource.com/312440 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tryjob-Request: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Tested-by: Olli Etuaho <oetuaho@nvidia.com>