Encapsulate all of the implementation inside the .cpp file, and pass
just the diagnostics object instead of the whole ParseContext to the
validation function.

BUG=angleproject:1670
TEST=angle_unittests

Change-Id: I89713b63e554dbedaa12b2270208f1fac496c54e
Reviewed-on: https://chromium-review.googlesource.com/420788Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>