A nested extension pipeline like this

$unionWith: {from: coll, pipeline: [{$facet: {facetPipe: [{$extensionStage: {}}]}}]}

throws an invariant (if we remove options.makeSubpipelineOwned) when the extension tries calling getOriginalBson() most likely because DocumentSourceFacet::extractRawPipelines() doesn't make copies and makes the BSON go out of scope.

We should make $facet handle the BSON properly to avoid making unnecessary makeOwned() calls from the LPP ctor.