HLSL: Fix a grammar error related to constructors in parenthetical expressions

Test/hlsl.init2.frag

@@ -0,0 +1,23 @@
+
+void Test1()
+{
+    struct mystruct { float2 a; };
+    mystruct test1 = {
+        { 1, 2, },          // test trailing commas
+    };
+
+    mystruct test2 = {
+        { { 1, 2, } },      // test unneeded levels
+    };
+
+    float test3 = { 1 } ;   // test scalar initialization
+}
+
+struct PS_OUTPUT { float4 color : SV_Target0; };
+
+PS_OUTPUT main()
+{
+    PS_OUTPUT ps_output;
+    ps_output.color = 1.0;
+    return ps_output;
+}