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

[Arvind Mukund via cfe-commits]

MasterAwesome wrote:

@AaronBallman, @erichkeane; whenever ya'll are done with the review, can you merge this in? I don't think I have access. And about the backporting, do we need it for 17.x bugfix releases?

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