Added checks for overlapping union initializers, as shown in bug #259 .

tests/mini/nocompile_initoverlap1.d

@@ -0,0 +1,8 @@
+struct Vector {
+        union { float x; float y; }
+        const static Vector zero = { x : 0, y : 0 };
+}
+
+struct HBoxLayout {
+        Vector padding  = Vector.zero;
+}