Rollup merge of #102232 - Urgau:stabilize-bench_black_box, r=TaKO8Ki

Stabilize bench_black_box

This PR stabilize `feature(bench_black_box)`.

```rust
pub fn black_box<T>(dummy: T) -> T;
```

The FCP was completed in https://github.com/rust-lang/rust/issues/64102.

`@rustbot` label +T-libs-api -T-libs

compiler/rustc_index/src/lib.rs

@@ -1,7 +1,6 @@
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
 #![feature(allow_internal_unstable)]
-#![feature(bench_black_box)]
 #![feature(extend_one)]
 #![feature(min_specialization)]
 #![feature(new_uninit)]