Rollup merge of #134600 - dtolnay:chainedcomparison, r=oli-obk

Fix parenthesization of chained comparisons by pretty-printer

Example:

```rust
macro_rules! repro {
    () => {
        1 < 2
    };
}

fn main() {
    let _ = repro!() == false;
}
```

Previously `-Zunpretty=expanded` would pretty-print this syntactically invalid output: `fn main() { let _ = 1 < 2 == false; }`

```console
error: comparison operators cannot be chained
 --> <anon>:8:23
  |
8 | fn main() { let _ = 1 < 2 == false; }
  |                       ^   ^^
  |
help: parenthesize the comparison
  |
8 | fn main() { let _ = (1 < 2) == false; }
  |                     +     +
```

With the fix, it will print `fn main() { let _ = (1 < 2) == false; }`.

Making `-Zunpretty=expanded` consistently produce syntactically valid Rust output is important because that is what makes it possible for `cargo expand` to format and perform filtering on the expanded code.

## Review notes

According to `rg '\.fixity\(\)' compiler/` the `fixity` function is called only 3 places:

- https://github.com/rust-lang/rust/blob/13170cd787cb733ed24842ee825bcbd98dc01476/compiler/rustc_ast_pretty/src/pprust/state/expr.rs#L283-L287

- https://github.com/rust-lang/rust/blob/13170cd787cb733ed24842ee825bcbd98dc01476/compiler/rustc_hir_pretty/src/lib.rs#L1295-L1299

- https://github.com/rust-lang/rust/blob/13170cd787cb733ed24842ee825bcbd98dc01476/compiler/rustc_parse/src/parser/expr.rs#L282-L289

The 2 pretty printers definitely want to treat comparisons using `Fixity::None`. That's the whole bug being fixed. Meanwhile, the parser's `Fixity::None` codepath is previously unreachable as indicated by the comment, so as long as `Fixity::None` here behaves exactly the way that `Fixity::Left` used to behave, you can tell that this PR definitely does not constitute any behavior change for the parser.

My guess for why comparison operators were set to `Fixity::Left` instead of `Fixity::None` is that it's a very old workaround for giving a good chained comparisons diagnostic (like what I pasted above). Nowadays that is handled by a different dedicated codepath.

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