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Issue Description:

Summary:
When I subclass a class defined in another module, the “Inherits from” relationship field is listed on the page but its blank.

The `targetFallback` does appear in the symbol graph like it does for protocol conformance, so this appears to be a DocC bug, not a symbol graph bug.

I'd expect the behavior for subclassing here to match the behavior for protocol conformance.

"relationships": [
    {
        "kind": "conformsTo",
        "source": "s:3Foo0A5ClassC",
        "target": "s:3Bar0A8ProtocolP",
        "targetFallback": "Bar.BarProtocol"
    },
    {
        "kind": "inheritsFrom",
        "source": "s:3Foo0A5ClassC",
        "target": "s:3Bar0A5ClassC",
        "targetFallback": "Bar.BarClass"
    }
]

Notes:
Attaching the full symbol graph for this project along with a reproducing Swift package.