This is an intermediate step to only storing a TVariable * in
TIntermSymbol instead of copying the name.

This makes it possible to get a constant value out of a TIntermSymbol
without doing a symbol table lookup.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: Ibff588241a4ad4ac330063296273288b20a072c9
Reviewed-on: https://chromium-review.googlesource.com/829142
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>