Follow up to #197311 and based on the feedback on that PR.
One way a Python synthetic formatter can construct a child is with
`SBValue::GetSyntheticChildAtOffset`. However those values were are not
marked as having been generated by `SyntheticChildren`. This means the
previous PR (#197311) did not apply to them as would be expected. The
fix is to call `SetSyntheticChildrenGenerated(true)` on the child values
created in `GetSyntheticChildAtOffset`.
Assisted-by: claude (tests and review)
Dave Lee
b6a56c4665