Auto merge of #125051 - dtolnay:printletelse, r=compiler-errors

Pretty-print let-else with added parenthesization when needed

Rustc used to produce invalid syntax for the following code, which is problematic because it means we cannot apply rustfmt to the output of `-Zunpretty=expanded`.

```rust
macro_rules! expr {
    ($e:expr) => { $e };
}

fn main() {
    let _ = expr!(loop {}) else { return; };
}
```

```console
$ rustc repro.rs -Zunpretty=expanded | rustfmt
error: `loop...else` loops are not supported
 --> <stdin>:9:29
  |
9 | fn main() { let _ = loop {} else { return; }; }
  |                     ----    ^^^^^^^^^^^^^^^^
  |                     |
  |                     `else` is attached to this loop
  |
  = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
```

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