Adjust `llvm_unreachable` macro to account for platforms that don't define LLVM_BUILTIN_UNREACHABLE

Post 892c104f, LLVM_BUILTIN_UNREACHABLE may not be defined anymore.
Also when LLVM_UNREACHABLE_OPTIMIZE is OFF, emit LLVM_BUILTIN_UNREACHABLE
after LLVM_BUILTIN_TRAP to ensure that diagnostics are suppressed on
environments where LLVM_BUILTIN_TRAP is not marked as noreturn.

Differential Revision: https://reviews.llvm.org/D122170