Enable GL_KHR_shader_subgroup properly

(and GL_NV_shader_subgroup_partitioned) based on GL/ES version
instead of predicating it on vulkan SPV generation

Also add AST testing.
The glsl.450.subgroup* files are largely the same as the spv.subgroup*
The glsl.es320.subgroup* files are the same as the 450 versions, but modified to be ES compatible.

Test/glsl.450.subgroup.vert

@@ -0,0 +1,11 @@
+#version 450
+#extension GL_KHR_shader_subgroup_basic: enable
+layout(set = 0, binding = 0, std430) buffer Output
+{
+  uvec4 result[];
+};
+
+void main (void)
+{
+  result[gl_VertexID] = uvec4(gl_SubgroupSize, gl_SubgroupInvocationID, 0, 0);
+}