Filter out events from analysis when constructing execution-phase values in Skyframe.

This avoids some unnecessary iteration over already-emitted events that can show up in profiles, and allows us to store execution-phase values a bit more compactly, since we don't need to carry around wrapper objects and nested sets everywhere.

This crucially depends on the fact that we can't build a target in the execution phase without first having analyzed it in a separate Skyframe call. Skyframe normally propagates all events/posts up the graph because it must be able to emit them if a user requests a node that only transitively depends on the node that emitted an event. However, because we do analysis in a separate Skyframe call, any warnings/posts associated with the analysis nodes will be emitted then, and we don't need to propagate them into execution.

PiperOrigin-RevId: 208767078