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

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

I mean that `stddef.h` would be modified to provide it as a function instead of a macro. In C++ it would simply never be a macro.

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