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/ty_test/src/lib.rs

@@ -6,10 +6,7 @@ use colored::Colorize;
 use config::SystemKind;
 use parser as test_parser;
 use ruff_db::Db as _;
-use ruff_db::diagnostic::{
-    Diagnostic, DisplayDiagnosticConfig, create_parse_diagnostic,
-    create_unsupported_syntax_diagnostic,
-};
+use ruff_db::diagnostic::{Diagnostic, DisplayDiagnosticConfig};
 use ruff_db::files::{File, system_path_to_file};
 use ruff_db::panic::catch_unwind;
 use ruff_db::parsed::parsed_module;
@@ -325,14 +322,14 @@ fn run_test(
             let mut diagnostics: Vec<Diagnostic> = parsed
                 .errors()
                 .iter()
-                .map(|error| create_parse_diagnostic(test_file.file, error))
+                .map(|error| Diagnostic::syntax_error(test_file.file, &error.error, error))
                 .collect();
 
             diagnostics.extend(
                 parsed
                     .unsupported_syntax_errors()
                     .iter()
-                    .map(|error| create_unsupported_syntax_diagnostic(test_file.file, error)),
+                    .map(|error| Diagnostic::syntax_error(test_file.file, error, error)),
             );
 
             let mdtest_result = attempt_test(db, check_types, test_file, "run mdtest", None);