Fixed const qualifiers on named structs/unions members that should prevent assignments to the whole structs/unions.

Added warning on ignored qualifiers on anonymous structs/unions.

test/err/bug2018.c

@@ -0,0 +1,25 @@
+/* Bug #2018 - Compiler has problems with const struct fields */
+
+struct X {
+    struct {
+        int a;
+    } a;
+    union {
+        int a;
+        const int b;
+    } b;
+};
+
+struct X f(void)
+{
+    struct X x = { 42 };
+    return x;
+}
+
+int main(void)
+{
+    struct X x = { 0 };
+    x = f();            /* Error since X is read only */
+
+    return 0;
+}