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/796795Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>