[nll] do a crater run with NLL enabled, track results

[nikomatsakis]

It'd be good for us to do a crater run with NLL enabled by default and try to track the results. One problem is that it's not clear if the compiler will bootstrap with NLL enabled (#51823). In order to disconnect these two questions, we came up with the following strategy in a brief IRC discussion:

• Make a build of rustc where -Zborrowck=mir is the default but only if the RUSTC_BOOTSTRAP environment variable is not set.
• Open a [WIP] PR with this branch and do a crater run.

After some discussion on IRC, w

[nikomatsakis]

Zulip thread can be found here

[Mark-Simulacrum]

We've started a crater run with ef9a322, this run is master vs. master with -Zborrowck=mir on one. Results should be available in around ~3 days.

[Mark-Simulacrum]

@lqd is working through the two crater runs we've done so far. It also turned out that we should enable -Ztwo-phase-borrows as well; we've now done so. This is the latest crater run: https://cargobomb-reports.s3.amazonaws.com/nll-3/index.html.

[nikomatsakis]

This has been done. @lqd and @Mark-Simulacrum are :superheros: