message: Implement `black_box` using intrinsic
  The new implementation allows some `memcpy`s to be optimized away,
  so the uninit value in ui/sanitize/memory.rs is constructed directly
  onto the return place. Therefore the sanitizer now says that the
  value is allocated by `main` rather than `random`.
diff: @@ -1136,6 +1136,11 @@ fn type_by_size(size: Size) -> Option<Type> {
  };
  ret.write_cvalue(fx, CValue::by_val(is_eq_value, ret.layout()));
  };
  +
  + black_box, (c a) {
  + // FIXME implement black_box semantics
  + ret.write_cvalue(fx, a);
  + };
  }
  if let Some((_, dest)) = destination {
  