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

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> (for example, we now include `string.h` and `stdbit.h` in freestanding, both of which provide a lot of function interfaces)

We do? I don't see those in lib/Headers

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