Auto merge of #55069 - matthewjasper:explain-free-region-liveness, r=nikomatsakis
[NLL] Use new region infer errors when explaining borrows
Use the new free region infer errors for explaining borrows
This gives at least some explanation for why a borrow is expected to
last for a certain free region. Also:
* Reports E0373: "closure may outlive the current function" with NLL.
* Special cases the case of returning a reference to (or value referencing) a local variable or temporary (E0515).
* Special case assigning a reference to a local variable in a closure to a captured variable. (E0521)
Closes #51026 - `regions-nested-fns-2.rs` isn't changed to that diagnostic, since that would not be the correct error here.
Closes #51169
cc #53882 - The error is (IMO) better now, but it could be better when we trace lifetimes in these error messages.
r? @nikomatsakis cc @pnkfelix