glr: fix improperly placed synclines

Predicates with GLR are issued with synclines in the middle of C code:

          case 2:
      if (! (#line 6 "sempred.y" /* glr.c:816  */
      new_syntax)) YYERROR;
    #line 793 "sempred.tab.c" /* glr.c:816  */
      break;

Reported by Rici Lake.
http://lists.gnu.org/archive/html/bug-bison/2018-05/msg00033.html

* data/c.m4 (b4_predicate_case): Be sure to start on column 0.
It would be nicer if b4_syncline could ensure this by itself
(that would avoid ugly code when synclines are disabled), but that's
way more work.
* tests/glr-regression.at (Predicates): Be a real end-to-end test.
This would have caught this error years ago...

data/c.m4

@@ -459,7 +459,8 @@ b4_syncline([@oline@], [@ofile@])
 # ------------------------------------
 m4_define([b4_predicate_case],
 [  case $1:
-    if (! ($2)) YYERROR;
+    if (! (
+$2)) YYERROR;
 b4_syncline([@oline@], [@ofile@])
     break;])