Rollup merge of #93556 - dtolnay:trailingcomma, r=cjgillot

Change struct expr pretty printing to match rustfmt style

This PR backports trailing comma support from https://github.com/dtolnay/prettyplease into rustc_ast_pretty and uses it to improve the formatting of struct expressions.

Example:

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

fn main() {
    println!("{}", stringify_expr!(Struct {
        a: Struct { b, c },
    }));
    println!("{}", stringify_expr!(Struct {
        aaaaaaaaaa: AAAAAAAAAA,
        bbbbbbbbbb: Struct {
            cccccccccc: CCCCCCCCCC,
            dddddddddd: DDDDDDDDDD,
            eeeeeeeeee: EEEEEEEEEE,
        },
    }));
}
```

🤮 Before:

```console
Struct{a: Struct{b, c,},}
Struct{aaaaaaaaaa: AAAAAAAAAA,
    bbbbbbbbbb:
        Struct{cccccccccc: CCCCCCCCCC,
            dddddddddd: DDDDDDDDDD,
            eeeeeeeeee: EEEEEEEEEE,},}
```

After:

```console
Struct { a: Struct { b, c } }
Struct {
    aaaaaaaaaa: AAAAAAAAAA,
    bbbbbbbbbb: Struct {
        cccccccccc: CCCCCCCCCC,
        dddddddddd: DDDDDDDDDD,
        eeeeeeeeee: EEEEEEEEEE,
    },
}
```

0 files changed, 0 insertions, 0 deletions