Rollup merge of #126883 - dtolnay:breakvalue, r=fmease

Parenthesize break values containing leading label

The AST pretty printer previously produced invalid syntax in the case of `break` expressions with a value that begins with a loop or block label.

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

fn main() {
    loop {
        break expr!('a: loop { break 'a 1; } + 1);
    };
}
```

`rustc -Zunpretty=expanded main.rs `:

```console
#![feature(prelude_import)]
#![no_std]
#[prelude_import]
use ::std::prelude::rust_2015::*;
#[macro_use]
extern crate std;
macro_rules! expr { ($e:expr) => { $e }; }

fn main() { loop { break 'a: loop { break 'a 1; } + 1; }; }
```

The expanded code is not valid Rust syntax. Printing invalid syntax is bad because it blocks `cargo expand` from being able to format the output as Rust syntax using rustfmt.

```console
error: parentheses are required around this expression to avoid confusion with a labeled break expression
 --> <anon>:9:26
  |
9 | fn main() { loop { break 'a: loop { break 'a 1; } + 1; }; }
  |                          ^^^^^^^^^^^^^^^^^^^^^^^^
  |
help: wrap the expression in parentheses
  |
9 | fn main() { loop { break ('a: loop { break 'a 1; }) + 1; }; }
  |                          +                        +
```

This PR updates the AST pretty-printer to insert parentheses around the value of a `break` expression as required to avoid this edge case.

Author: matthiaskrgr