Use TLValueTrackingTraverser to determine whether a loop index is used
as an l-value. This replaces custom logic in ValidateLimitations,
greatly simplifying the code. Also pass the symbol table to
ValidateLimitations as a parameter, which removes the need to store a
global pointer to the current ParseContext.

BUG=angleproject:1960
TEST=angle_unittests, WebGL conformance tests

Change-Id: I122c85c78bbea05833d7c787cd184de568c5c45f
Reviewed-on: https://chromium-review.googlesource.com/465606
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>