diagnostics: factor and enhance messages about duplicate rule directives

When reporting a duplicate directive on a rule, point to its first
occurrence:

one.y:11.10-15: error: only one %empty allowed per rule
   %empty {} %empty
             ^^^^^^
one.y:11.3-8: previous declaration
   %empty {} %empty
   ^^^^^^

And consistently discard the second one.

* src/complain.h, src/complain.c (duplicate_directive): New.
* src/reader.c: Use it where appropriate.
* src/symlist.h, src/symlist.c (symbol_list): Add a dprec_location member.
* tests/actions.at: Adjust expected output.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/complain.c b/src/complain.c
index 44e1b1ea..2e87097f 100644
--- a/src/complain.c
+++ b/src/complain.c
@@ -365,3 +365,13 @@ deprecated_directive (location const *loc, char const *old, char const *upd)
               _("deprecated directive: %s, use %s"),
               quote (old), quote_n (1, upd));
 }
+
+void
+duplicate_directive (char const *directive,
+                     location first, location second)
+{
+  unsigned i = 0;
+  complain (&second, complaint, _("only one %s allowed per rule"), directive);
+  i += SUB_INDENT;
+  complain_indent (&first, complaint, &i, _("previous declaration"));
+}