HLSL: Do structure conversion for return type struct-punning on non-entry-point functions.

Test/hlsl.multiReturn.frag

@@ -0,0 +1,19 @@
+struct S {
+    float f;
+    float3 v;
+    float3x3 m;
+};
+
+cbuffer bufName {
+    S s;
+};
+
+S foo()
+{
+    return s;
+}
+
+void main()
+{
+    foo();
+}