This patch intends to implement Geometry Shader layout qualifiers
required in OpenGL ES 3.1 extension GL_OES_geometry_shader in ANGLE
GLSL compiler.

1. Add support to the shader type GL_GEOMETRY_SHADER_OES.
2. Implement Geometry Shader layout qualifiers in the GLSL compiler:
(1) Add support to OpenGL ES 3.1 extension "GL_OES_geometry_shader".
(2) Add validations of the input and output primitive declarations
    in the Geometry Shader layout declarations.
(3) Add 'invocations' and 'max_vertices' support in the Geometry
    Shader layout declarations
3. Add unit tests to cover all the new features added in this patch.

BUG=angleproject:1941
TEST=angle_unittests

Change-Id: Ie693e11f8a00dab3552626ed63e9336c7fbd3cb8
Reviewed-on: https://chromium-review.googlesource.com/560647
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>