Linker: Eliminate uncalled functions, because they can be ill-defined.

Fixes issue #610. Also provides a testing option to keep uncalled functions.

Test/hlsl.deadFunctionMissingBody.vert

@@ -0,0 +1,8 @@
+float4 main(): SV_Target0 { return 0; }
+struct Surface { float3 albedo; };
+Surface surfaceShader(float fade);
+Surface surfaceShaderExec()
+{
+    float fade = 0;
+    return surfaceShader(0);
+}