Combine OldDiagnostic and Diagnostic (#19053)

## Summary

This PR is a collaboration with @AlexWaygood from our pairing session
last Friday.

The main goal here is removing `ruff_linter::message::OldDiagnostic` in
favor of
using `ruff_db::diagnostic::Diagnostic` directly. This involved a few
major steps:

- Transferring the fields
- Transferring the methods and trait implementations, where possible
- Converting some constructor methods to free functions
- Moving the `SecondaryCode` struct
- Updating the method names

I'm hoping that some of the methods, especially those in the
`expect_ruff_*`
family, won't be necessary long-term, but I avoided trying to replace
them
entirely for now to keep the already-large diff a bit smaller.

### Related refactors

Alex and I noticed a few refactoring opportunities while looking at the
code,
specifically the very similar implementations for
`create_parse_diagnostic`,
`create_unsupported_syntax_diagnostic`, and
`create_semantic_syntax_diagnostic`.
We combined these into a single generic function, which I then copied
into
`ruff_linter::message` with some small changes and a TODO to combine
them in the
future.

I also deleted the `DisplayParseErrorType` and `TruncateAtNewline` types
for
reporting parse errors. These were added in #4124, I believe to work
around the
error messages from LALRPOP. Removing these didn't affect any tests, so
I think
they were unnecessary now that we fully control the error messages from
the
parser.

On a more minor note, I factored out some calls to the
`OldDiagnostic::filename`
(now `Diagnostic::expect_ruff_filename`) function to avoid repeatedly
allocating
`String`s in some places.

### Snapshot changes

The `show_statistics_syntax_errors` integration test changed because the
`OldDiagnostic::name` method used `syntax-error` instead of
`invalid-syntax`
like in ty. I think this (`--statistics`) is one of the only places we
actually
use this name for syntax errors, so I hope this is okay. An alternative
is to
use `syntax-error` in ty too.

The other snapshot changes are from removing this code, as discussed on

[Discord](1388252408):


34052a1185/crates/ruff_linter/src/message/mod.rs (L128-L135)

I think both of these are technically breaking changes, but they only
affect
syntax errors and are very narrow in scope, while also pretty
substantially
simplifying the refactor, so I hope they're okay to include in a patch
release.

## Test plan

Existing tests, with the adjustments mentioned above

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff_linter/src/message/github.rs

@@ -1,9 +1,10 @@
 use std::io::Write;
 
+use ruff_db::diagnostic::Diagnostic;
 use ruff_source_file::LineColumn;
 
 use crate::fs::relativize_path;
-use crate::message::{Emitter, EmitterContext, OldDiagnostic};
+use crate::message::{Emitter, EmitterContext};
 
 /// Generate error workflow command in GitHub Actions format.
 /// See: [GitHub documentation](https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions#setting-an-error-message)
@@ -14,12 +15,13 @@ impl Emitter for GithubEmitter {
     fn emit(
         &mut self,
         writer: &mut dyn Write,
-        diagnostics: &[OldDiagnostic],
+        diagnostics: &[Diagnostic],
         context: &EmitterContext,
     ) -> anyhow::Result<()> {
         for diagnostic in diagnostics {
-            let source_location = diagnostic.compute_start_location();
-            let location = if context.is_notebook(&diagnostic.filename()) {
+            let source_location = diagnostic.expect_ruff_start_location();
+            let filename = diagnostic.expect_ruff_filename();
+            let location = if context.is_notebook(&filename) {
                 // We can't give a reasonable location for the structured formats,
                 // so we show one that's clearly a fallback
                 LineColumn::default()
@@ -27,7 +29,7 @@ impl Emitter for GithubEmitter {
                 source_location
             };
 
-            let end_location = diagnostic.compute_end_location();
+            let end_location = diagnostic.expect_ruff_end_location();
 
             write!(
                 writer,
@@ -35,7 +37,7 @@ impl Emitter for GithubEmitter {
                 code = diagnostic
                     .secondary_code()
                     .map_or_else(String::new, |code| format!(" ({code})")),
-                file = diagnostic.filename(),
+                file = filename,
                 row = source_location.line,
                 column = source_location.column,
                 end_row = end_location.line,
@@ -45,7 +47,7 @@ impl Emitter for GithubEmitter {
             write!(
                 writer,
                 "{path}:{row}:{column}:",
-                path = relativize_path(&*diagnostic.filename()),
+                path = relativize_path(&filename),
                 row = location.line,
                 column = location.column,
             )?;