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/250380Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>