-Wempty-rule: diagnose empty rules without %empty

* src/complain.h, src/complain.c (warning_empty_rule, Wempty_rule):
New warning category.
(warnings_args, warnings_types): Adjust.
* src/reader.c (grammar_rule_check): Check the empty rules are
flagged by %empty.
* tests/actions.at (Implicitly empty rule): New.
* tests/existing.at: Add expected warnings.

tests/actions.at

@@ -64,6 +64,28 @@ AT_PARSER_CHECK([./input], 0,
 
 AT_CLEANUP
 
+## ----------------------- ##
+## Implicitly empty rule.  ##
+## ----------------------- ##
+
+AT_SETUP([Implicitly empty rule])
+
+AT_DATA_GRAMMAR([[1.y]],
+[[%%
+exp: a b;
+a: /* empty. */ {};
+// A mid-rule action does not count as an empty rule.
+b: {} {};
+]])
+
+AT_BISON_CHECK([-fcaret -Wempty-rule 1.y], [0], [],
+[[1.y:11.17-18: warning: empty rule without %empty [-Wempty-rule]
+ a: /* empty. */ {};
+                 ^^
+]])
+
+AT_CLEANUP
+
 ## ------------------------ ##
 ## Invalid uses of %empty.  ##
 ## ------------------------ ##