Rollup merge of #92487 - dtolnay:traitalias, r=matthewjasper

Rollup merge of #92487 - dtolnay:traitalias, r=matthewjasper

Fix unclosed boxes in pretty printing of TraitAlias

This was causing trait aliases to not even render at all in stringified / pretty printed output.

```rust
macro_rules! repro {
    ($item:item) => {
        stringify!($item)
    };
}

fn main() {
    println!("{:?}", repro!(pub trait Trait<T> = Sized where T: 'a;));
}
```

Before:&ensp;`""`
After:&ensp;`"pub trait Trait<T> = Sized where T: 'a;"`

The fix is copied from how `head`/`end` for `ItemKind::Use`, `ItemKind::ExternCrate`, and `ItemKind::Mod` are all done in the pretty printer:

https://github.com/rust-lang/rust/blob/dd3ac41495e85a9b7b5cb3186379d02ce17e51fe/compiler/rustc_ast_pretty/src/pprust/state.rs#L1178-L1184