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

[Aaron Ballman via cfe-commits]

AaronBallman 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

Sorry for the confusion! "We" being WG14 (the C committee). We (Clang) haven't done anything about either of these headers as we're expecting the user to have to find a freestanding runtime library that provides them.

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