Auto merge of #42924 - pnkfelix:mir-dataflow, r=arielb1

Shift mir-dataflow from `rustc_borrowck` to `rustc_mir` crate.

Shift mir-dataflow from `rustc_borrowck` to `rustc_mir` crate.

Turn `elaborate_drops` and `rustc_peek` implementations into MIR passes that also live in `rustc_mir` crate.

Rewire things so `rustc_driver` uses the `ElaborateDrops` from `rustc_mir` crate.

(This PR is another baby step for mir-borrowck; it is a piece of work that other people want to rebase their stuff on top of, namely developers who are doing other dataflow analyses on top of MIR.)

I have deliberately architected this PR in an attempt to minimize the number of actual code changes. The majority of the diff should be little more than changes to mod and use declarations, as well as a few visibility promotions to pub(crate) when a declaration was moved downward in the module hierarchy.

(I have no problem with other PR's that move declarations around to try to clean this up; my goal was to ensure that the diff here was as small as possible, to make the review nearly trivial.)

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