Clean up GLSL switch statement validation 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/420788 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>