patch 9.0.0623: error for modifying a const is not detected at compile time

Problem:    Error for modifying a const is not detected at compile time.
Solution:   Add TTFLAG_CONST and check for it in add() and extend().
diff --git a/src/structs.h b/src/structs.h
index 27fb5bd..4caff2d 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1445,10 +1445,10 @@
     type_T	*type_decl;	    // declared type or equal to type_current
 } type2_T;
 
-#define TTFLAG_VARARGS	1	    // func args ends with "..."
-#define TTFLAG_OPTARG	2	    // func arg type with "?"
-#define TTFLAG_BOOL_OK	4	    // can be converted to bool
-#define TTFLAG_STATIC	8	    // one of the static types, e.g. t_any
+#define TTFLAG_VARARGS	0x01	    // func args ends with "..."
+#define TTFLAG_BOOL_OK	0x02	    // can be converted to bool
+#define TTFLAG_STATIC	0x04	    // one of the static types, e.g. t_any
+#define TTFLAG_CONST	0x08	    // cannot be changed
 
 /*
  * Structure to hold an internal variable without a name.