This makes the order of the output always consistent:
1. Place of the `match` missing arms
2. The `enum` definition span
3. The structured suggestion to add a fallthrough arm
* On suggestions that include deletions, use a diff inspired output format
* When suggesting addition, use `+` as underline
* Color highlight modified span
Previously, we threw away the `Span` associated with a definition's
identifier when we encoded crate metadata, causing us to lose location
and hygiene information.
We now store the identifier's `Span` in the crate metadata.
When we decode items from the metadata, we combine
the name and span back into an `Ident`.
This improves the output of several tests, which previously had messages
suppressed due to dummy spans.
This is a prerequisite for #68686, since throwing away a `Span` means
that we lose hygiene information.
