Get action warnings (grammar_rule_check) right even when symbol

declarations appear after the rules.  Don't mistake the type of $$ in
a midrule to be that of its parent rule's $$.
* src/reader.c (grammar_current_rule_end): Don't invoke
grammar_rule_check yet since not all symbol declarations may have been
parsed yet.
(grammar_midrule_action): Likewise.
Don't record whether the midrule's $$ has been used yet since actions
haven't been translated yet.
Record the midrule's parent rule and its RHS index within the parent
rule.
(grammar_current_rule_action_append): Don't translate the action yet
since not all symbol declarations may have been parsed yet and, thus,
warnings about types for $$, $n, @$, and @n can't be reported yet.
(packgram): Translate the action and invoke grammar_rule_check now that
all symbol declarations have been parsed.
* src/scan-code.l (handle_action_dollar): Now that this is invoked
after parsing the entire grammar file, the symbol list here in the case
of a midrule is actually the midrule's empty RHS, so reference its
parent rule's RHS where necessary.
On the other hand, now that you can already know it's a midrule, you
aren't forced to think $$ has the same type as its parent rule's $$.
(handle_action_at): In the case of a midrule, reference the parent rule
where necessary.
* src/symlist.c (symbol_list_new): Initialize new midrule-related
members.
(symbol_list_length): Now that this is invoked after all rules have
been parsed, a NULL symbol (rather than a NULL symbol list node)
terminates a rule.  symbol_list_print already does this correctly.
* src/symlist.h (symbol_list.midrule_parent_rule,
symbol_list.midrule_parent_rhs_index): New members so that midrules can
remember their relationships with their parents.
* tests/input.at (Type Clashes): Extend to catch the midrule $$ error
fixed by the above patch.
(_AT_UNUSED_VALUES_DECLARATIONS, AT_CHECK_UNUSED_VALUES): New m4 macros
implementing...
(Unused values): ... this old test case and...
(Unused values before symbol declarations): ... this new test case.
This one is the same as `Unused values' except that all symbol
declarations appear after the rules in order to catch the rest of the
errors fixed by the above patch.

src/symlist.h

@@ -32,10 +32,17 @@ typedef struct symbol_list
   symbol *sym;
   location location;
 
-  /* If this symbol is the generated lhs for a mid-rule, a pointer to
-     that mid-rule.  */
+  /* If this symbol is the generated lhs for a midrule but this is the rule in
+     whose rhs it appears, MIDRULE = a pointer to that midrule.  */
   struct symbol_list *midrule;
 
+  /* If this symbol is the generated lhs for a midrule and this is that
+     midrule, MIDRULE_PARENT_RULE = a pointer to the rule in whose rhs it
+     appears, and MIDRULE_PARENT_RHS_INDEX = its rhs index (1-origin) in the
+     parent rule.  */
+  struct symbol_list *midrule_parent_rule;
+  int midrule_parent_rhs_index;
+
   /* The action is attached to the LHS of a rule. */
   const char *action;
   location action_location;