[C2x] Implement the `unreachable` macro for WG14 N2826

This exposes __builtin_unreachable as the expansion for the unreachable
macro in C2x. I added this definition under __need_STDDEF_H_misc on the
assumption there is no need for a separate need macro to control adding
this.

Differential Revision: https://reviews.llvm.org/D143430