Reset symbol unique id counter between compilations This guarantees identical compilation results on different compilations using the same compiler instance, guards against overflow, and is useful as a building block for tracking more symbol information in the symbol table. BUG=angleproject:2267 TEST=angle_unittests Change-Id: Ib5a7cec2fff6712ead969d935d238d28a87fd4a7 Reviewed-on: https://chromium-review.googlesource.com/796795 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>