Implement parsing switch statements Put in some groundwork for parsing switch statements and case labels in the parser, including definitions for IntermNode classes. Intermediate functions for adding the statements are stubbed to only generate errors for now. Tested by manually disabling shading language version checks for switch in a Chromium build and checking that the expected errors are generated. BUG=angle:921 Change-Id: I064b3e0c4c1b724a083cf5bc78eebfdd3794eb1b Reviewed-on: https://chromium-review.googlesource.com/250380 Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Tested-by: Olli Etuaho <oetuaho@nvidia.com>