Auto merge of #43108 - pnkfelix:mir-borrowck3c, r=arielb1

MIR borrow check (under debug flag)

Here is the current state of MIR borrow check.

It consists of (1.) some refactoring, (2.) a dataflow analysis to identify the borrows themselves, and (3.) a mir "transform" that does the borrow check itself based on the aforementioned dataflow results.

(There's also a drive-by fix to dataflow that I can factor into a separate PR if necessary. Interestingly I could not find a way to observe the bug outside of MIR borrowck.)

To be clear, this branch is not ready to be used as the default borrow check. Thus the code is guarded: To get mir-borrowck to run, you need to either supply an attribute `#[rustc_mir_borrowck]` or a debug flag `-Z borrowck-mir`.

Here are the main issues with the current MIR borrowck as it stands in this PR:

 * No Notes emitted yet, just errors. (So the feedback is definitely inferior compared to AST borrowck today)
 * Lvalue rendering differs between Ast and Mir. (Mostly minor, but replacement of field names with indices is very bad; big priority for me to fix ASAP.)
 * Lots of ICEs (presumably because some MIR operations used here have well-formedness assumptions that are violated in borrowck-broken code)
 * Conflates lots of cases that are distinguished by AST-borrowck
 * Conflates "uninitialized" with "moved" (special case of previous bullet, one that I think should be fixed ASAP)

 (I am hoping to fix as many of the above issues as I can in the near term, but I also would like to land this even if they are *not* all fixed, because the rebasing effort is getting to be a real drag.)

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