Move evsel::leader to perf_evsel::leader, so we can move the group
interface to libperf.

Also add several evsel helpers to ease up the transition:

  struct evsel *evsel__leader(struct evsel *evsel);
  - get leader evsel

  bool evsel__has_leader(struct evsel *evsel, struct evsel *leader);
  - true if evsel has leader as leader

  bool evsel__is_leader(struct evsel *evsel);
  - true if evsel is itw own leader

  void evsel__set_leader(struct evsel *evsel, struct evsel *leader);
  - set leader for evsel

Committer notes:

Fix this when building with 'make BUILD_BPF_SKEL=1'

  tools/perf/util/bpf_counter.c

  -       if (evsel->leader->core.nr_members > 1) {
  +       if (evsel->core.leader->nr_members > 1) {

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Requested-by: Shunsuke Nakamura <nakamura.shun@fujitsu.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Mark Rutland <mark.rutland@arm.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/20210706151704.73662-4-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>