HLSL: enable #line extension by default for HLSL source.

Test/hlsl.pp.line.frag

@@ -0,0 +1,24 @@
+
+#line 1
+
+struct PS_OUTPUT
+{
+    float4 Color : SV_Target0;
+    float  Depth : SV_Depth;
+};
+
+#line 2
+
+PS_OUTPUT main()
+{
+   PS_OUTPUT psout;
+
+#line 123 "SomeFile.frag"
+
+   int thisLineIs = __LINE__;  // gets 124
+
+   psout.Color = float4(thisLineIs, 0, 0, 1);
+   psout.Depth = 1.0;
+
+   return psout;
+}