This will make it easier to generate errors and warnings with a
consistent format outside of ParseContext, for example in
TIntermBinary::fold() which warns about division by zero.

BUG=angleproject:952
TEST=angle_unittests

Change-Id: I25999d7bdc77efafe77785a0d6f4d63417dfaab9
Reviewed-on: https://chromium-review.googlesource.com/372719Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>