Move most of the logic in glslang.y to ParseContext. This will make it
easier to change the code in the future.

Only a few specific bits of logic are kept in glslang.y:

* Disabling a parsing rule when a given shading language version is
  being parsed. This makes it easier to check the grammar against the
  grammar in the GLSL ES specs.
* Scoping calls that need to be paired with another call. It's much
  easier to check these for correctness when the paired calls are next
  to each other.

BUG=angleproject:911
TEST=angle_unittests

Change-Id: I52f42a1fc0f28463ca4b237dc6e88345e5173064
Reviewed-on: https://chromium-review.googlesource.com/539640
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>