Auto merge of #72256 - ecstatic-morse:once-cell, r=Mark-Simulacrum

Use `once_cell` crate instead of custom data structure

Internally, we use the [`Once`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/sync/struct.Once.html) type for shared data that is initialized exactly once and only read from afterwards. `Once` uses a `parking_lot::Mutex` when the parallel compiler is enabled and a `RefCell` when it is not. This PR switches to the [`once_cell`](https://crates.io/crates/once_cell) crate, which also uses a `parking_lot::Mutex` for its `sync` version (because we enable the `parking_lot` feature) but has zero overhead for its `unsync` one.

This PR adds `once_cell` to the list of whitelisted dependencies. I think this is acceptable because it is already used in `rustc_driver`, is owned by a well-known community member (cc @matklad), and has a stable release. cc @rust-lang/compiler

`once_cell` has a slightly more minimal API than `Once`, which allows for initialization to be either optimistic (evaluate the initializer and then synchronize) or pessimistic (synchronize and then evaluate the initializer). `once_cell`'s `get_or_init` is always pessimistic. The optimistic version is only used once in the current `master`.

r? @Mark-Simulacrum

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