Unify `OldDiagnostic` and `Message` (#18391)
Summary
--
This PR unifies the remaining differences between `OldDiagnostic` and
`Message` (`OldDiagnostic` was only missing an optional `noqa_offset`
field) and
replaces `Message` with `OldDiagnostic`.
The biggest functional difference is that the combined `OldDiagnostic`
kind no
longer implements `AsRule` for an infallible conversion to `Rule`. This
was
pretty easy to work around with `is_some_and` and `is_none_or` in the
few places
it was needed. In `LintContext::report_diagnostic_if_enabled` we can
just use
the new `Violation::rule` method, which takes care of most cases.
Most of the interesting changes are in [this
range](https://github.com/astral-sh/ruff/pull/18391/files/8156992540f4e81f914a9534e99241b1e14382a1)
before I started renaming.
Test Plan
--
Existing tests
Future Work
--
I think it's time to start shifting some of these fields to the new
`Diagnostic`
kind. I believe we want `Fix` for sure, but I'm less sure about the
others. We
may want to keep a thin wrapper type here anyway to implement a `rule`
method,
so we could leave some of these fields on that too.
