[3.11] gh-105497: [Enum] Fix Flag inversion when alias/mask members exist. (GH-105542) (#105571)

When inverting a Flag member (or boundary STRICT), only consider other canonical flags; when inverting an IntFlag member (or boundary KEEP), also consider aliases.
(cherry picked from commit 59f009e5898a006cdc8f5249be589de6edfe5cd0)

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>