Rollup merge of #120158 - jubnzv:120130-mirdf-exports, r=nnethercote

`rustc_mir_dataflow`: Restore removed exports

Added back previously available exports:

* `Forward`/`Backward`: used when implementing `AnalysisDomain`
* `Engine`: used in user's code to solve the dataflow problem
* `SwitchIntEdgeEffects`: used when implementing functions of the `Analysis` trait
* `graphviz`: potentially useful for debugging purposes

Closes #120130