diagnostics: update the grammar file

Let's use the fixits to actually update the grammar files.

* src/getargs.h, src/getargs.c (update_flag): New.
* src/fixits.h, src/fixits.c (fixits_run): New.
* src/main.c (main): Invoke fixits_run when --update is passed.
* tests/input.at (Deprecated directives): Check --update.

src/fixits.h

@@ -23,6 +23,9 @@
 /* Declare a fix to apply.  */
 void fixits_register (location const *loc, char const* update);
 
+/* Apply the fixits: update the source file.  */
+void fixits_run (void);
+
 /* Free the registered fixits.  */
 void fixits_free (void);