Rollup merge of #69644 - ecstatic-morse:unified-dataflow-cleanup, r=eddyb

Remove framework in `dataflow/mod.rs` in favor of "generic" one

This is the culmination of the work described in rust-lang/compiler-team#202. All dataflow analyses (including the one in `clippy`) have been ported to use the framework in `dataflow/generic`, which can efficiently handle both gen/kill and generic problems. This PR moves the framework in `dataflow/generic` to `dataflow/framework`, and removes the gen/kill framework in `dataflow/mod.rs`.

More comprehensive documentation for the new framework is tracked in rust-lang/rustc-guide#564.

`clippy` will need to change the path it uses to import the dataflow analysis traits.