Rollup merge of #98854 - kadiwa4:rustc_hir_pretty_clean_up_borrowing, r=oli-obk

clean up the borrowing in rustc_hir_pretty

A whole lot of the `&`s and `ref`s were redundant. I hope doing this in one big commit is fine, because all of the changes are pretty self-contained.

`@rustbot` label: +C-cleanup