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

[Aaron Ballman via cfe-commits]

AaronBallman 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.

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