perf expr: Force encapsulation on expr_id_data

perf expr: Force encapsulation on expr_id_data

This patch resolves some undefined behavior where variables in
expr_id_data were accessed (for debugging) without being defined. To
better enforce the tagged union behavior, the struct is moved into
expr.c and accessors provided. Tag values (kinds) are explicitly
identified.

Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-By: Kajol Jain<kjain@linux.ibm.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Link: https://lore.kernel.org/r/20200826153055.2067780-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>