Rollup merge of #121377 - pitaj:lazy_cell_fn_pointer, r=dtolnay

Stabilize `LazyCell` and `LazyLock`

Closes #109736

This stabilizes the [`LazyLock`](https://doc.rust-lang.org/stable/std/sync/struct.LazyLock.html) and [`LazyCell`](https://doc.rust-lang.org/stable/std/cell/struct.LazyCell.html) types:

```rust
static HASHMAP: LazyLock<HashMap<i32, String>> = LazyLock::new(|| {
    println!("initializing");
    let mut m = HashMap::new();
    m.insert(13, "Spica".to_string());
    m.insert(74, "Hoyten".to_string());
    m
});

let lazy: LazyCell<i32> = LazyCell::new(|| {
    println!("initializing");
    92
});
```

r? libs-api

src/tools/clippy/clippy_dev/src/lib.rs

@@ -1,4 +1,3 @@
-#![feature(lazy_cell)]
 #![feature(let_chains)]
 #![feature(rustc_private)]
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]

src/tools/clippy/src/driver.rs

@@ -2,7 +2,6 @@
 #![allow(rustc::untranslatable_diagnostic)]
 #![feature(rustc_private)]
 #![feature(let_chains)]
-#![feature(lazy_cell)]
 #![feature(lint_reasons)]
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
 // warn on lints, that are included in `rust-lang/rust`s bootstrap

src/tools/clippy/tests/compile-test.rs

@@ -1,4 +1,3 @@
-#![feature(lazy_cell)]
 #![feature(is_sorted)]
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
 #![warn(rust_2018_idioms, unused_lifetimes)]

src/tools/clippy/tests/dogfood.rs

@@ -3,7 +3,6 @@
 //!
 //! See [Eating your own dog food](https://en.wikipedia.org/wiki/Eating_your_own_dog_food) for context
 
-#![feature(lazy_cell)]
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
 #![warn(rust_2018_idioms, unused_lifetimes)]
 

src/tools/clippy/tests/lint_message_convention.rs

@@ -1,4 +1,3 @@
-#![feature(lazy_cell)]
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
 #![warn(rust_2018_idioms, unused_lifetimes)]
 

src/tools/clippy/tests/workspace.rs

@@ -1,5 +1,3 @@
-#![feature(lazy_cell)]
-
 use std::path::PathBuf;
 use std::process::Command;
 use test_utils::{CARGO_CLIPPY_PATH, IS_RUSTC_TEST_SUITE};