Fix unclosed boxes in pretty printing of TraitAlias

[dtolnay]

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

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

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

Before: ""
After: "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:

rust/compiler/rustc_ast_pretty/src/pprust/state.rs

Lines 1178 to 1184 in dd3ac41

	ast::ItemKind::Use(ref tree) => {
	self.head(visibility_qualified(&item.vis, "use"));
	self.print_use_tree(tree);
	self.word(";");
	self.end(); // end inner head-block
	self.end(); // end outer head-block
	}

[rust-highfive]

r? @matthewjasper

(rust-highfive has picked a reviewer for you, use r? to override)

[matthewjasper]

@bors r+

[bors]

📌 Commit a18f43f has been approved by matthewjasper

[dtolnay]

@bors rollup