TIntermSymbol nodes are now constructed based on a specific TVariable.
This makes sure that all TIntermSymbol nodes that are created to refer
to a specific temporary in an AST transform will have consistent data.
The TVariable objects are not necessarily added to the symbol table
levels - just those variables that can be referred to by their name
during parsing need to be reachable through there.

In the future this can be taken a step further so that TIntermSymbol
nodes just to point to a TVariable instead of duplicating the
information.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I4e7bcdb0637cd3b588d3c202ef02f4b7bd7954a1
Reviewed-on: https://chromium-review.googlesource.com/811925
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>