perf annotate-data: Add annotated_data_type__get_member_name()

perf annotate-data: Add annotated_data_type__get_member_name()

Factor out a function to get the name of member field at the given
offset.  This will be used in other places.

Also update the output of typeoff sort key a little bit.  As we know
that some special types like (stack operation), (stack canary) and
(unknown) won't have fields, skip printing the offset and field.

For example, the following change is expected.

  "(stack operation) +0 (no field)"   ==>   "(stack operation)"

Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250310224925.799005-2-namhyung@kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>