Auto merge of #126958 - dtolnay:u32char, r=Mark-Simulacrum

Stabilize const unchecked conversion from u32 to char

Closes rust-lang/rust#89259.

The functions in this PR were left out of the initial set of `feature(const_char_convert)` stabilizations in rust-lang/rust#102470, but have since been unblocked by rust-lang/rust#118979.

If `unsafe { from_u32_unchecked(u) }` is called in const with a value for which `from_u32(u)` returns None, we get the following compile error.

```rust
fn main() {
    let _ = const { unsafe { char::from_u32_unchecked(0xd800) } };
}
```

```console
error[E0080]: it is undefined behavior to use this value
 --> src/main.rs:2:19
  |
2 |     let _ = const { unsafe { char::from_u32_unchecked(0xd800) } };
  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered 0x0000d800, but expected a valid unicode scalar value (in `0..=0x10FFFF` but not in `0xD800..=0xDFFF`)
  |
  = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
  = note: the raw bytes of the constant (size: 4, align: 4) {
              00 d8 00 00                                     │ ....
          }

note: erroneous constant encountered
 --> src/main.rs:2:13
  |
2 |     let _ = const { unsafe { char::from_u32_unchecked(0xd800) } };
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

Author: bors