By using this abstraction rather than accessing the properties directly, we smooth the transition to https://github.com/endojs/endo/pull/1712, tolerating both the legacy and current guard shapes.

Unlike LegacyAwaitArgGuardShape, tolerating LegacyMethodGuardShape does not seem like a currently exploitable bug, because there is not currently any context where either a methodGuard or a copyRecord would both be meaningful.