Orphaned BP actors when using UWorldPartitionBuilder

From an EPS case:

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We're seeing a recurring issue where child BP actors are becoming orphaned during any processing that uses UWorldPartitionBuilder.

It affects cases where both the parent and child are BP actors, and we've narrowed the problem down to how AActor::RerunConstructionScripts() is invoked by ULevel::AddLoadedActors() as part of the processing done by UWorldPartitionBuilder::Run().

Simply, if AActor::RerunConstructionScripts() happens to run on the parent actor before the child, the AttachParent of the child actor's root component is left pointing to a component that has been marked for GC, which results in the child actor having a nullptr for its AttachParent should it then get saved as a consequence of the processing job.

The impact of this is that actors are becoming detached and moved to the origin en masse during overnight processing jobs, which is having a destructive effect on project productivity.

The issue seems similar to problems we experienced with UWorldPartition::ApplyRuntimeCellsTransformerStack() - in that case, we were able to apply these upcoming 5.7 fixes to address the issue, but so far I haven't seen anything equivalent for UWorldPartitionBuilder -

https://github.com/EpicGames/UnrealEngine/commit/f11b2502b2e15f37333a80b55d0227756c0b4df1

https://github.com/EpicGames/UnrealEngine/commit/c02114f28a258cae849a33988927cfb186455235