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/baseResults/spv.subgroupSizeARB.frag.out

@@ -0,0 +1,34 @@
+spv.subgroupSizeARB.frag
+// Module Version 10000
+// Generated by (magic number): 8000a
+// Id's are bound by 12
+
+                              Capability Shader
+                              Capability SubgroupBallotKHR
+                              Extension  "SPV_KHR_shader_ballot"
+               1:             ExtInstImport  "GLSL.std.450"
+                              MemoryModel Logical GLSL450
+                              EntryPoint Fragment 4  "main" 8 10
+                              ExecutionMode 4 OriginUpperLeft
+                              Source GLSL 450
+                              SourceExtension  "GL_ARB_shader_ballot"
+                              SourceExtension  "GL_KHR_shader_subgroup_basic"
+                              Name 4  "main"
+                              Name 8  "result"
+                              Name 10  "gl_SubGroupSizeARB"
+                              Decorate 8(result) Location 0
+                              Decorate 10(gl_SubGroupSizeARB) Flat
+                              Decorate 10(gl_SubGroupSizeARB) BuiltIn SubgroupSize
+               2:             TypeVoid
+               3:             TypeFunction 2
+               6:             TypeInt 32 0
+               7:             TypePointer Output 6(int)
+       8(result):      7(ptr) Variable Output
+               9:             TypePointer Input 6(int)
+10(gl_SubGroupSizeARB):      9(ptr) Variable Input
+         4(main):           2 Function None 3
+               5:             Label
+              11:      6(int) Load 10(gl_SubGroupSizeARB)
+                              Store 8(result) 11
+                              Return
+                              FunctionEnd

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;
+}