This PR makes `AnalysisData` and`BorrowckCtxt` public. Those types are returned by the public function `build_borrowck_dataflow_data_for_fn` and are needed if a caller wants to pass on the return values.
It also removes `FnPartsWithCFG`, which required callers of `build_borrowck_dataflow_data_for_fn` to have a reference to a `CFG` with the same lifetime as `FnParts`, which is more limiting than required.
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