Stabilize const unchecked conversion from u32 to char

[dtolnay]

Closes #89259.

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

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

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) } };
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[rustbot]

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[dtolnay]

@rustbot ready (completed FCP in the tracking issue)

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit 9d3c79b has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit 9d3c79b with merge fcaa6fd...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing fcaa6fd to master...

[rust-timer]

Finished benchmarking commit (fcaa6fd): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

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Bootstrap: 704.234s -> 705.014s (0.11%)
Artifact size: 328.65 MiB -> 328.67 MiB (0.00%)