…rovements, r=compiler-errors

Never type unsafe lint improvements

- Move linting code to a separate method
- Remove mentions of `core::convert::absurd` (rust-lang#124311 was rejected)
- Make the lint into FCW

The last thing is a bit weird though. On one hand it should be `EditionSemanticsChange(2024)`, but on the other hand it shouldn't, because we also plan to break it on all editions some time later. _Also_, it's weird that we don't have `FutureReleaseSemanticsChangeReportInDeps`, IMO "this might cause UB in a future release" is important enough to be reported in deps...

IMO we ought to have three enums instead of [`FutureIncompatibilityReason`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint_defs/enum.FutureIncompatibilityReason.html#):

```rust
enum IncompatibilityWhen {
     FutureRelease,
     Edition(Edition),
}

enum IncompatibilyWhat {
    Error,
    SemanticChange,
}

enum IncompatibilityReportInDeps {
    No,
    Yes,
}
```

Tracking:
- rust-lang#123748

Rollup merge of rust-lang#125282 - WaffleLapkin:never-type-unsafe-improvements, r=compiler-errors

Never type unsafe lint improvements

- Move linting code to a separate method
- Remove mentions of `core::convert::absurd` (rust-lang#124311 was rejected)
- Make the lint into FCW

The last thing is a bit weird though. On one hand it should be `EditionSemanticsChange(2024)`, but on the other hand it shouldn't, because we also plan to break it on all editions some time later. _Also_, it's weird that we don't have `FutureReleaseSemanticsChangeReportInDeps`, IMO "this might cause UB in a future release" is important enough to be reported in deps...

IMO we ought to have three enums instead of [`FutureIncompatibilityReason`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint_defs/enum.FutureIncompatibilityReason.html#):

```rust
enum IncompatibilityWhen {
     FutureRelease,
     Edition(Edition),
}

enum IncompatibilyWhat {
    Error,
    SemanticChange,
}

enum IncompatibilityReportInDeps {
    No,
    Yes,
}
```

Tracking:
- rust-lang#123748