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

[Louis Dionne via cfe-commits]

ldionne 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.

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