HLSL: Add EHTokStringConstant, so that string attributes may be parsed

Test/hlsl.stringtoken.frag

@@ -0,0 +1,20 @@
+
+struct PS_OUTPUT
+{
+    float4 Color : SV_Target0;
+};
+
+Texture2D TestTexture <
+    string TestAttribute1 = "TestAttribute";
+    string TestAttribute2 = "false";
+    int    TestAttribute3 = 3;
+>;
+
+uniform float4 TestUF <string StrValue = "foo";>;
+
+PS_OUTPUT main()
+{
+   PS_OUTPUT psout;
+   psout.Color = float4(0,0,0,1);
+   return psout;
+}