Rollup merge of rust-lang#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`

Author: matthiaskrgr

compiler/rustc_abi/src/lib.rs

@@ -3,6 +3,7 @@
 #![cfg_attr(feature = "nightly", doc(rust_logo))]
 #![cfg_attr(feature = "nightly", feature(rustdoc_internals))]
 #![cfg_attr(feature = "nightly", feature(step_trait))]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use std::fmt;

compiler/rustc_arena/src/lib.rs

@@ -25,6 +25,7 @@
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
 #![feature(strict_provenance)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use std::alloc::Layout;