Fixed constness of bit-fields.

test/err/bug2018-bitfield.c

@@ -0,0 +1,14 @@
+/* Bug #2018 - Compiler has problems with const struct fields */
+
+typedef union U {
+    int a : 16;
+    const int b : 16;
+} U;
+
+int main(void)
+{
+    U x = { 42 };
+    x.b = 0;
+
+    return 0;
+}