Rollup merge of #126013 - nnethercote:unreachable_pub, r=Urgau

Add `#[warn(unreachable_pub)]` to a bunch of compiler crates

By default `unreachable_pub` identifies things that need not be `pub` and tells you to make them `pub(crate)`. But sometimes those things don't need any kind of visibility. So they way I did these was to remove the visibility entirely for each thing the lint identifies, and then add `pub(crate)` back in everywhere the compiler said it was necessary. (Or occasionally `pub(super)` when context suggested that was appropriate.) Tedious, but results in more `pub` removal.

There are plenty more crates to do but this seems like enough for a first PR.

r? `@compiler-errors`

compiler/rustc_codegen_llvm/src/back/archive.rs

@@ -86,7 +86,7 @@ impl<'a> ArchiveBuilder for LlvmArchiveBuilder<'a> {
     }
 }
 
-pub struct LlvmArchiveBuilderBuilder;
+pub(crate) struct LlvmArchiveBuilderBuilder;
 
 impl ArchiveBuilderBuilder for LlvmArchiveBuilderBuilder {
     fn new_archive_builder<'a>(&self, sess: &'a Session) -> Box<dyn ArchiveBuilder + 'a> {