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

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> > Is that still worth mention in the release notes do you think?
> 
> Oh duh, that's right, this requires someone to define the `__need` macro to get it, so no, I don't think it needs a release note. Sorry for the noise!

👍 thanks for bearing with me on this!

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