The GLSL parser now allows a runtime-sized array as the last member in
a shader storage block. Clamping indexing against the memory bounds is
done by determining the array length at runtime.

Runtime-sized arrays are used in dEQP tests for many compute shader
tests, so these now work on the OpenGL backend.

BUG=angleproject:1951
TEST=angle_unittests,
     dEQP-GLES31.functional.shaders.linkage.shader_storage_block.*
     dEQP-GLES31.functional.shaders.builtin_functions.*compute*

Change-Id: Ibecca24623ca8e4723af6f0e0421fe9711ea828d
Reviewed-on: https://chromium-review.googlesource.com/787976
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>