Merge pull request #276 from Unity-Technologies/fix-subgraph-name-start-with-number

[FIX] #272 This will prefix the generated code with "sg_"

com.unity.shadergraph/Editor/Data/Nodes/Utility/SubGraphNode.cs

         private string SubGraphFunctionName()
         {
             var functionName = subGraphAsset != null ? NodeUtils.GetHLSLSafeName(subGraphAsset.name) : "ERROR";
-            return string.Format("{0}_{1}", functionName, GuidEncoder.Encode(referencedGraph.guid));
+            return string.Format("sg_{0}_{1}", functionName, GuidEncoder.Encode(referencedGraph.guid));
         }
 
         public virtual void GenerateNodeFunction(FunctionRegistry registry, GenerationMode generationMode)