We have made a lot of progress on NLL diagnostics

So much so that it behooves us to take a step back and reevaluate where we stand with respect to overall diagnostic quality.

As part of that, we should do another round of review of the differences between AST borrowck diagnostic output and NLL borrowck diagnostic output.

• see NLL: must review all .nll.stderr files #49862 for the previous round of review here

Update: I finished the review, and have put the cases where NLL is not up to par into "buckets." Each bucket has a linked comment on this issue (and, in some cases, a separate issue that is linked from that comment and from the description here).

Here are all the buckets (with links to each specific comment):

• When a bucket is no longer relevant (either because the issue has been fixed or we've decided its not actually an issue), it will be struckout (like so).