[clang] [Headers] Don't declare unreachable() from stddef.h in C++ (PR #86748)

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> LGTM, though we should have a release note about the change because we've been exposing the macro since Clang 17. I don't think this warrant a potentially breaking change notice, though, just a regular bugfix one.

I think we weren't exposing `unreachable` in C++ at all in Clang 17, and it's only exposed in Clang 18 if something sets the new `__needs_unreachable`? I can still release note it if you think it's worth calling out.

https://github.com/llvm/llvm-project/pull/86748