If parsing fails then destructors are ran to clean the up the stack.
Rename the head union member to make the term and evlist use cases more
distinct, this simplifies matching the correct destructor.

Committer notes:

Jiri: "Nice did not know about this.. looks like it's been in bison for some time, right?"

Ian:  "Looks like it wasn't in Bison 1 but in Bison 2, we're at Bison 3 and
       Bison 2 is > 14 years old:
       https://web.archive.org/web/20050924004158/http://www.gnu.org/software/bison/manual/html_mono/bison.html#Destructor-Decl"
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: bpf@vger.kernel.org
Cc: clang-built-linux@googlegroups.com
Cc: netdev@vger.kernel.org
Link: http://lore.kernel.org/lkml/20191030223448.12930-7-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>