message: NFC: Don't print the location of a diagnostic if it is unknown.
  Printing 'loc(unknown)' just clutters the output with unhelpful information.
diff: @@ -400,11 +400,13 @@ void SourceMgrDiagnosticHandler::emitDiagnostic(Location loc, Twine message,
  // Extract a file location from this loc.
  auto fileLoc = getFileLineColLoc(loc);
  - // If one doesn't exist, then print the location as part of the message.
  + // If one doesn't exist, then print the raw message without a source location.
  if (!fileLoc) {
  std::string str;
  llvm::raw_string_ostream strOS(str);
  - strOS << loc << ": " << message;
  + if (!loc.isa<UnknownLoc>())
  + strOS << loc << ": ";
  + strOS << message;
  return mgr.PrintMessage(os, llvm::SMLoc(), getDiagKind(kind), strOS.str());
  }
  