In warnings, say "previous declaration" rather than "first

declaration".
* src/symtab.c (redeclaration): Do that here.
* src/reader.c (record_merge_function_type): In the case of a result
type clash, report the previous declaration rather than the very first
one in the grammar file.
* tests/glr-regression.at (Missed %merge type warnings when LHS type is
declared later): Add a third declaration to check this behavior.
* tests/input.at (Incompatible Aliases): Update output.

src/symtab.c

@@ -108,7 +108,7 @@ static void
 redeclaration (symbol* s, const char *what, location first, location second)
 {
   complain_at (second, _("%s redeclaration for %s"), what, s->tag);
-  complain_at (first, _("first declaration"));
+  complain_at (first, _("previous declaration"));
 }