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/312440Reviewed-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>