Rollup merge of #148358 - makai410:rpub-test-fix, r=celinval

Fix some issues around `rustc_public`

cc rust-lang/rust#148266 .
follow-up of rust-lang/rust#148341 .

This fixes the issues that can be reproduced by `x test compiler/rustc_public`:

```
error: function `run` is never used
   --> compiler/rustc_public/src/compiler_interface.rs:838:15
    |
838 | pub(crate) fn run<'tcx, F, T>(interface: &CompilerInterface<'tcx>, f: F) -> Result<T, Error>
    |               ^^^
    |
    = note: `-D dead-code` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(dead_code)]`

error: unreachable `pub` item
  --> compiler/rustc_public/src/unstable/mod.rs:25:1
   |
25 | pub trait InternalCx<'tcx>: Copy + Clone {
   | ---^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   | |
   | help: consider restricting its visibility: `pub(crate)`
   |
   = help: or consider exporting it for use by other crates
   = note: `-D unreachable-pub` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unreachable_pub)]`

error: unreachable `pub` item
  --> compiler/rustc_public/src/unstable/mod.rs:62:1
   |
62 | pub trait Stable<'tcx>: PointeeSized {
   | ---^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   | |
   | help: consider restricting its visibility: `pub(crate)`
   |
   = help: or consider exporting it for use by other crates

error: unreachable `pub` item
  --> compiler/rustc_public/src/unstable/mod.rs:81:1
   |
81 | pub trait RustcInternal {
   | ---^^^^^^^^^^^^^^^^^^^^
   | |
   | help: consider restricting its visibility: `pub(crate)`
   |
   = help: or consider exporting it for use by other crates

error: could not compile `rustc_public` (lib) due to 4 previous errors
```

Author: Zalathar