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

[Aaron Ballman via cfe-commits]

AaronBallman 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!

Likewise! :-)

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