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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Why do the Clang builtin headers even try to define this? Shouldn't this be provided by the platform's C library instead? I am really confused about what's the intended layering here and it seems to me like Clang is overstepping its responsibilities by basically implementing parts of the C library.

I did that in a9797d7d2d7878464868312e0c147b4e747a31c6

It's a freestanding feature in C23, so we need to expose it from `stddef.h` at least when not in hosted builds. I don't understand why this is making into C++ builds at all: 

We set `__need_unreachable` based on `__STDC_VERSION__`:
https://github.com/llvm/llvm-project/blob/c388690a8b96cbdfa8c38a1e050088201da648e5/clang/lib/Headers/stddef.h#L59

We only include `__stddef_unreachable.h` if `__need_unreachable` is defined:
https://github.com/llvm/llvm-project/blob/c388690a8b96cbdfa8c38a1e050088201da648e5/clang/lib/Headers/stddef.h#L102


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