HLSL: allow GS-specific methods in other stages

Using GS methods such as Append() in non-GS stages should be ignored, but was
creating errors due to the lack of a stream output symbol for the non-GS stage.

Test/hlsl.fraggeom.frag

@@ -0,0 +1,17 @@
+// test geometry shader in fragment shader.  GS attributes should be successfully ignored.
+
+struct myVertex {
+    float4 pos : SV_Position;
+};
+
+[maxvertexcount(1)]
+void GS_Draw(point myVertex IN, inout PointStream<myVertex> OutputStream)
+{
+    OutputStream.Append(IN);
+    OutputStream.RestartStrip();
+}
+
+float4 main() : SV_TARGET
+{
+    return 0;
+}