PP/HLSL: Fix #1424: support comma in nested curly braces for macro arg

Test/hlsl.pp.expand.frag

@@ -0,0 +1,17 @@
+#define EMP1(a)
+#define EMP2(a, b)
+
+#define EXP1(a) = a
+#define EXP2(a, b) = a, b
+
+struct A
+{
+    float4 a EMP1({1,2,3,4});                           // No PP arg errors
+    float4 b EMP2({({{(({1,2,3,4}))}})}, {{1,2,3,4}});  // No PP arg errors
+    float4 c EXP1({1,2,3,4});                           // ERROR: No PP arg errors, but init error
+    float4 d EXP2({({{(({1,2,3,4}))}})}, {{1,2,3,4}});  // ERROR: No PP arg errors, but init error
+};
+
+void main()
+{
+}