Reformat use declarations.

The previous commit updated `rustfmt.toml` appropriately. This commit is
the outcome of running `x fmt --all` with the new formatting options.

compiler/rustc_ast_pretty/src/helpers.rs

@@ -1,6 +1,7 @@
-use crate::pp::Printer;
 use std::borrow::Cow;
 
+use crate::pp::Printer;
+
 impl Printer {
     pub fn word_space<W: Into<Cow<'static, str>>>(&mut self, w: W) {
         self.word(w);