HLSL: fix preprocessor concatenation behaviour.

Fix #772.

Test/hlsl.semantic-1.vert

@@ -0,0 +1,24 @@
+#define DLAYER 3
+
+#define DMACRO1 TEXCOORD1
+#define DMACRO(num) TEXCOORD##num
+
+struct S {
+	float4 pos	: POSITION;
+	float2 UV0    	: TEXCOORD0;
+	float2 UV1	: DMACRO1;
+	float2 UV2	: DMACRO(2);
+	float2 UV3	: DMACRO(DLAYER);
+};
+
+
+S main(float4 v : POSITION)
+{
+    S s;
+    s.pos = v;
+    s.UV0 = float2(v.x,v.x);
+    s.UV1 = float2(v.y,v.y);
+    s.UV2 = float2(v.z,v.z);
+    s.UV3 = float2(v.w,v.w);
+    return s;
+}