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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Oh wow, that bug has been around for quite a while. I like your solution to it!

The code LGTM, no tests required because there isn't really a decent way to test this from lit. However, it would be nice to add a release note to `clang/docs/ReleateNotes.rst` to tell users that the bug was fixed. I'd add the note to the `Bug Fixes in This Version` section in the document.

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