Rollup merge of #109373 - ids1024:llvm-unreachable-optimize, r=ozkanonur

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.

Author: matthiaskrgr