Auto merge of #6194 - dwijnand:todo-remove-Compilation.native_dirs, r=dwijnand

Remove the remote TODO on Compilation.native_dirs

It's still used, see tests:

* build_script::doctest_receives_build_link_args
* build_script::rename_with_link_search_path
* run::run_with_library_paths
* run::library_paths_sorted_alphabetically

Originally I was trying to action https://github.com/rust-lang/cargo/pull/792#discussion_r226826431 with this PR