gl_WorkGroupSize should be written into the AST as a constant node
instead of a symbol node. In correct shaders, local size is guaranteed
to have been declared before any references to gl_WorkGroupSize -
otherwise the shader translator was already generating an error.

This ensures that work group size can be used to size arrays as
specified and also works around a crash issue on NVIDIA Linux OpenGL
driver.

BUG=angleproject:1442
TEST=angle_unittests

Change-Id: I9b1a4bff16ecf2c3db1511c3702756346cdd2f6b
Reviewed-on: https://chromium-review.googlesource.com/418735
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>