perf stat: Fail on extra comma while parsing events

Ian reported that we allow to parse following:

  $ perf stat -e ,cycles true

which is wrong and we should fail, like we do with this fix:

  $ perf stat -e ,cycles true
  event syntax error: ',cycles'
                        \___ parser error

The reason is that we don't have rule for ',' in 'event' start condition
and it's matched and accepted by default rule.

Add scanner debug support (that Ian already added for expr code),
which was really useful for finding this. It's enabled together with
bison debug via 'make PARSER_DEBUG=1'.

Reported-by: Ian Rogers <irogers@google.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200520074050.156988-1-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/parse-events.c

@@ -2041,6 +2041,7 @@ static int parse_events__scanner(const char *str, void *parse_state, int start_t
 
 #ifdef PARSER_DEBUG
 	parse_events_debug = 1;
+	parse_events_set_debug(1, scanner);
 #endif
 	ret = parse_events_parse(parse_state, scanner);