Auto merge of #117472 - jmillikin:stable-c-str-literals, r=Nilstrieb

Stabilize C string literals

RFC: https://rust-lang.github.io/rfcs/3348-c-str-literal.html

Tracking issue: rust-lang/rust#105723

Documentation PR (reference manual): rust-lang/reference#1423

# Stabilization report

Stabilizes C string and raw C string literals (`c"..."` and `cr#"..."#`), which are expressions of type [`&CStr`](https://doc.rust-lang.org/stable/core/ffi/struct.CStr.html). Both new literals require Rust edition 2021 or later.

```rust
const HELLO: &core::ffi::CStr = c"Hello, world!";
```

C strings may contain any byte other than `NUL` (`b'\x00'`), and their in-memory representation is guaranteed to end with `NUL`.

## Implementation

Originally implemented by PR rust-lang/rust#108801, which was reverted due to unintentional changes to lexer behavior in Rust editions < 2021.

The current implementation landed in PR rust-lang/rust#113476, which restricts C string literals to Rust edition >= 2021.

## Resolutions to open questions from the RFC

* Adding C character literals (`c'.'`) of type `c_char` is not part of this feature.
  * Support for `c"..."` literals does not prevent `c'.'` literals from being added in the future.
* C string literals should not be blocked on making `&CStr` a thin pointer.
  * It's possible to declare constant expressions of type `&'static CStr` in stable Rust (as of v1.59), so C string literals are not adding additional coupling on the internal representation of `CStr`.
* The unstable `concat_bytes!` macro should not accept `c"..."` literals.
  * C strings have two equally valid `&[u8]` representations (with or without terminal `NUL`), so allowing them to be used in `concat_bytes!` would be ambiguous.
* Adding a type to represent C strings containing valid UTF-8 is not part of this feature.
  * Support for a hypothetical `&Utf8CStr` may be explored in the future, should such a type be added to Rust.