Make gl_SubGroupARB a flat int in Vulkan

gl_SubGroupARB was being correctly declared as an Input variable in
Vulkan but it was missing the Flat decoration, which made spirv-val emit
the VUID-StandaloneSpirv-Flat-04744 validation error with shaders using
that built-in.

Test/spv.subgroupSizeARB.frag

@@ -0,0 +1,10 @@
+#version 450
+#extension GL_ARB_shader_ballot : enable
+#extension GL_KHR_shader_subgroup_basic : enable
+
+layout(location = 0) out uint result;
+
+void main (void)
+{
+  result = gl_SubGroupSizeARB;
+}