NLL: change compare-mode=nll to use borrowck=migrate

[pnkfelix]

We have a couple different modes that NLL can run in.

borrowck=mir is sort of the "adopt NLL whole hog"; it just runs the MIR-based borrow checker, and reports its errors as, well, errors.

borrowck=migrate is the migration plan for the 2018 edition. It runs the MIR-based borrow checker, and if it signals an error, then it runs the old AST-based borrow checker as a backup. If the code is accepted by AST-based borrow checker, then all the MIR-based errors are downgraded to warnings. This is the mode that we'll be deploying for Rust 2018.

Anyway: We are currently testing borrowck=mir for our test suite via compare-mode=nll.

The plan is to deploy borrowck=migrate.

We have barely any automated testing of borrowck=migrate.

See the problem here?

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My proposal: We should change compare-mode=nll to use borrowck=migrate instead of borrowck=mir. We should make this change ASAP.

[pnkfelix]

(The reason I hadn't proposed this earlier is that I had hoped that I would "simply" resolve issue #52979. But that turned out to be quite difficult in practice.)

[pnkfelix]

tagging with multiple NLL-labels because this kind of task overlaps all these buckets