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

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

I do wonder if we could have the broader builtin headers discussion independent of this patch? Is everyone happy with this patch? We can keep talking about the builtin headers in here independent of merging right?

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