Currently, the Clang Dataflow Framework only does Widen on backedges from structured loops. Missing some Widen calls (e.g., when there are backedges from gotos) could cause some analyses to iterate ~forever (until the max visits limit is hit). This adds a simple search for backedges, and triggers Widen on the additional backedge nodes. Fixes [issue 179083. ](https://github.com/llvm/llvm-project/issues/179083)