[clang] [LifetimeSafety] Use ImmutableList for buildOriginFlowChain DFS nodes (PR #208890)

[Utkarsh Saxena via cfe-commits]

https://github.com/usx95 commented:

I think the a list per `DFSNode` is the only bottleneck (quadratic). The rest of places (like `buildOriginFlowChain` return type) do not need `ImmutableList`s.
That would also keep the interfaces simpler, like not needing to have `OriginFlowChainFactory` as an arg. The factory can be local to multi-block buildOriginFlowChain

https://github.com/llvm/llvm-project/pull/208890