add llvm writable attribute

[quiode]

This PR depends on #154946. It tries to address rust-lang/unsafe-code-guidelines#584 (comment). It is part of a bachelor thesis supervised by @JoJoDeveloping and @RalfJung, for more information, see: Project_Description.pdf.
The llvm writable attribute is now inserted for all mutable borrows on function entry, if no_writable is not set. #154948 helps to find any new undefined behavior that is being introduced by this change.

[quiode]

r? @RalfJung

[rustbot]

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[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Executing "/scripts/stage_2_test_set1.sh"
+ /scripts/stage_2_test_set1.sh
PR_CI_JOB set; skipping tidy
+ '[' 1 == 1 ']'
+ echo 'PR_CI_JOB set; skipping tidy'
+ SKIP_TIDY='--skip tidy'
+ ../x.py --stage 2 test --skip tidy --skip compiler --skip src
##[group]Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
##[endgroup]
downloading https://static.rust-lang.org/dist/2026-03-05/rustfmt-nightly-aarch64-unknown-linux-gnu.tar.xz
---
To only update this specific test, also pass `--test-args cfg/suggest-alternative-name-on-target.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/cfg/suggest-alternative-name-on-target.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/cfg/suggest-alternative-name-on-target" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error: unexpected `cfg` condition value: `arm`
##[error]  --> /checkout/tests/ui/cfg/suggest-alternative-name-on-target.rs:5:7
   |
---
LL | #![deny(unexpected_cfgs)]
   |         ^^^^^^^^^^^^^^^
help: `arm` is an expected value for `target_arch`
   |
LL - #[cfg(target_abi = "arm")]
LL + #[cfg(target_arch = "arm")]
   |

error: unexpected `cfg` condition value: `gnu`
##[error]  --> /checkout/tests/ui/cfg/suggest-alternative-name-on-target.rs:12:7
   |
LL | #[cfg(target_arch = "gnu")]
   |       ^^^^^^^^^^^^^^^^^^^
   |
   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
help: `gnu` is an expected value for `target_env`
   |
LL - #[cfg(target_arch = "gnu")]
LL + #[cfg(target_env = "gnu")]
   |

error: unexpected `cfg` condition value: `openbsd`
##[error]  --> /checkout/tests/ui/cfg/suggest-alternative-name-on-target.rs:19:7
   |

[rust-bors]

☔ The latest upstream changes (presumably #154958) made this pull request unmergeable. Please resolve the merge conflicts.