NLL diagnostics has lost "closure may outlive the current function" notes

[pnkfelix]

The following ui/ tests have regressed in the quality of their diagnostic output (compared to AST borrowck, at least) due to the loss of notes of the form "closure may outlive the current function"

• borrowck/borrowck-escaping-closure-error-1.rs
• borrowck/borrowck-escaping-closure-error-2.rs
• issue-4335.stderr
• region-borrow-params-issue-29793-small.rs
• regions-nested-fns-2.rs "closure may outlive the current function" isn't correct here
• src/test/ui/issue-40510-3.nll.stderr
• src/test/ui/issue-49824.nll.stderr

(This list of tests is drawn from an informal paper document that I have been using to keep notes for myself as I work on this...)

[nikomatsakis]

I'm tagging this as RC though it seems related to the other region error issues described in #51027

[nikomatsakis]

Some comments on this starting about here on Zulip -- around here I highlighted how we might solve this

[matthewjasper]

Added the two extra cases that that discussion was about