message: Avoid failing when attempting to print null Attribute.
  This avoids segfaulting when dumping during debugging of failures.
diff: @@ -408,6 +408,11 @@ void ModulePrinter::printFunctionReference(const Function *func) {
  }
  void ModulePrinter::printAttribute(Attribute attr) {
  + if (!attr) {
  + os << "<<NULL ATTRIBUTE>>";
  + return;
  + }
  +
  switch (attr.getKind()) {
  case Attribute::Kind::Bool:
  os << (attr.cast<BoolAttr>().getValue() ? "true" : "false");
  