and update fact generation to the new relation names

A `Body` now contains its `MirSource`, which in turn contains the
`DefId` of the item associated with the `Body`.

This is only really useful in debug messages, so I've switched to
calling `span_to_string` in any place that causes a `Span` to end up in
user-visible output.

Fixes #69446

When we encounter a region error involving an `FnMut` closure, we
display a specialized error message. However, we currently do not
tell the user which upvar was captured. This makes it difficult to
determine the cause of the error, especially when the closure is large.

This commit records marks constraints involving closure upvars
with `ConstraintCategory::ClosureUpvar`. When we decide to 'blame'
a `ConstraintCategory::Return`, we additionall store
the captured upvar if we found a `ConstraintCategory::ClosureUpvar` in
the path.

When generating an error message, we point to relevant spans if we have
closure upvar information available. We further customize the message if
an `async` closure is being returned, to make it clear that the captured
variable is being returned indirectly.

- update polonius-engine dependency to 0.12.0

- rustfmt the files failing tidy

We want type checking for function bodies to ignore/erase regions. As
such, we need to infer the regions in opaque types in borrow check
instead.