add #[rustc_no_writable] to slice::get_unchecked_mut

[quiode]

This PR adds the #[rustc_no_writable] attribute introduced in #155207 to the slice::get_unchecked_mut function.

Two library functions already received this attribute, as they were known to cause problems with the llvm writable attribute and tree borrows. Since that PR, I ran Miri on the 30'000 most downloaded crates to see what kind of code is now UB under Tree Borrows + implicit writes, using the detection implemented in rust-lang/miri#4947. Adding this attribute to ignore checking for this function reduced the new UB introduced by more than 75%, meaning that instead of 19000 tests in 1700 crates having a difference, now only 3500 in 350 crates show a difference (measurement still running).

[rustbot]

r? @Mark-Simulacrum

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[quiode]

r? @RalfJung

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