The owning ID reference of RNA pointers is supposed to point at the
ID container that owns the object, but for PoseBone.bone it wasn't
updated to point at the Armature data ID instead of Armature Object.
This caused issues, like pose_bone.bone.driver_add() adding the
driver to the Armature Object animation data, which violates the
intended design of the animation data structures.
Since RNA code generally assumes that all pointers that don't
refer directly to an ID remain within the current ID, a custom
getter is required to fix this.
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