rustdoc: Allow private modules be included in docs

Made it so that what passes are used is passed onto the renderer so it
can intelligently deal with private modules.
auto merge of #18822 : scialex/rust/better-rustdoc, r=alexcrichton

Changed `rustdoc` so that if we do not have the `strip-private` pass
enabled private modules will be included in the generated documentation

I added this because it is useful to be able to read the documentation in the very nice `rustdoc` web interface when doing internal work on a project. In this case we want the `rustdoc` to include modules that are hidden from consumers. Since this is not currently possible I added it here.