Fix subgraph not being retained by SubgraphElements.retain()

[jcmosc]

In situations where the data passed to a ForEach is a _VariadicView.Children, as is the case for nested variadic views like so:

struct PassthroughUnaryViewRoot: _VariadicView.UnaryViewRoot {
    func body(children: _VariadicView.Children) -> some View {
        children
    }
}

_VariadicView.Tree(PassthroughUnaryViewRoot()) {
    ForEach(["A"], id: \.self) { element in
        Text(element)
    }
}

The subgraph for the inner for each item is released more than it is retained, causing the subgraph to invalidate and the view to render an empty list.

[Kyle-Ye]

LGTM.

[Kyle-Ye]

The ToggleUITests.onAndOffWithDefaultStyle is failing but I can reproduce it locally. It should pass after retry.

You can view detail and download failed image here https://github.com/OpenSwiftUIProject/OpenSwiftUI/actions/runs/19669127537?pr=653