[clang] [clang] Correct behavior of `LLVM_UNREACHABLE_OPTIMIZE=OFF` for `Release` builds (PR #68284)

[Arvind Mukund via cfe-commits]

MasterAwesome wrote:

> > > > @erichkeane @AaronBallman, I've added the release notes and rebased the change on `main`. Does this need to be backported to 17.x? 16.x doesn't have this issue
> > > 
> > > 
> > > Yeah, I think we should probably backport this to 17.x. We have instructions on how to kick that off here: https://www.llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches but I can get the ball rolling by adding the magic comment to the issue.
> > 
> > Thanks for doing it! I'll work on cherry-picking this to Rust's fork of llvm :)
> 
> You're welcome, but it seems there's a problem with trying to cherry-pick this over to 17.x and the pick will be more involved because of merge conflicts with the release notes. Would you mind taking over the pick to 17.x? If not, that's okay! (I'm mostly asking because I'm pretty swamped with reviews and I don't want the backport to fall through the cracks by accident.)

I gotchu! The PR created by llvmbot is at https://github.com/llvm/llvm-project-release-prs/pull/726

https://github.com/llvm/llvm-project/pull/68284