[Dependency Scanning] Emit diagnostics on scan query failure during initialization or cycle detection

[artemcm]

On dependency scan failure query, ensure any diagnostics that the compiler has accumulated along the way are still produced to the client.

For example, if scan query compilation instance initialization fails on an unknown argument to the frontend, the unknown argument diagnostic will be captured, and returned to the client.

Or, on encountering a dependency cycle, instead of returning an error to the client, a hollow result object is constructed carrying the diagnostic error state.

Resolves rdar://130897498