perf annotate: Introduce constructor/destructor for annotated_source

perf annotate: Introduce constructor/destructor for annotated_source

More stuff will go in there, all the parts that are not needed when a
symbol had no samples and that were mistakenly added to 'struct
annotation'.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-u4761kyzhixw9ydk6kib3f0o@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>