perf evlist: Introduce force_leader() method

perf evlist: Introduce force_leader() method

For non-explicit group (e.g. those created with -e '{eventA,eventB}'),
'perf report' supports a option '--group' which can enable group output.

We also need to support 'perf annotate' with the same '--group'.

Create a new function perf_evlist__force_leader() which contains common
code to force setting the group leader.

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1526914666-31839-2-git-send-email-yao.jin@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>