Set LLVM LLVM_UNREACHABLE_OPTIMIZE to OFF

This option was added to LLVM in
https://reviews.llvm.org/D121750?id=416339. It makes `llvm_unreachable`
in builds without assertions compile to an `LLVM_BUILTIN_TRAP` instead
of `LLVM_BUILTIN_UNREACHABLE` (which causes undefined behavior and is
equivalent to `std::hint::unreachable_unchecked`).

Having compiler bugs triggering undefined behavior generally seems
undesirable and inconsistent with Rust's goals. There is a check in
`src/tools/tidy/src/style.rs` to reject code using `llvm_unreachable`.
But it is used a lot within LLVM itself.

For instance, this changes a failure I get compiling `libcore` for m68k
from a `SIGSEGV` to `SIGILL`, which seems better though it still doesn't
provide a useful message without switching to an LLVM build with asserts.

It may be best not to do this if it noticeably degrades compiler
performance, but worthwhile if it doesn't do so in any significant way. I
haven't looked into what benchmarks there are for Rustc. That should be
considered before merging.

src/bootstrap/download-ci-llvm-stamp

@@ -1,4 +1,4 @@
 Change this file to make users of the `download-ci-llvm` configuration download
 a new version of LLVM from CI, even if the LLVM submodule hasn’t changed.
 
-Last change is for: https://github.com/rust-lang/rust/pull/104748
+Last change is for: https://github.com/rust-lang/rust/pull/109373

src/bootstrap/native.rs

@@ -309,6 +309,7 @@ impl Step for Llvm {
         cfg.out_dir(&out_dir)
             .profile(profile)
             .define("LLVM_ENABLE_ASSERTIONS", assertions)
+            .define("LLVM_UNREACHABLE_OPTIMIZE", "OFF")
             .define("LLVM_ENABLE_PLUGINS", plugins)
             .define("LLVM_TARGETS_TO_BUILD", llvm_targets)
             .define("LLVM_EXPERIMENTAL_TARGETS_TO_BUILD", llvm_exp_targets)