Add a message to mlir-opt when reading from stdin to avoid being waiting for nothing

[mehdi_amini]

Summary

It happens from time to time that one may run mlir-opt expecting something to
happen, but the process is waiting on stdin. Print a message when reading from
stdin to warn developers.

[mehdi_amini]

clang-format

[jpienaar]

Could this be done only if detected interactive session? E.g., when piping commands I'd prefer not to see this message (which is probably the majority of times when I use opt).

[mehdi_amini]

Detect terminal connected to stdin

[rriddle]

mlir/lib/Tools/mlir-opt/MlirOptMain.cpp

340–343

This deserves a comment of some kind.

mlir/lib/Tools/mlir-opt/MlirOptMain.cpp

std::string helpHeader = (toolName + "\nAvailable Dialects: ").str();
     llvm::raw_string_ostream os(helpHeader);
     interleaveComma(registry.getDialectNames(), os,
                     [&](auto name) { os << name; });
   }
   // Parse pass names in main to ensure static initialization completed.
   cl::ParseCommandLineOptions(argc, argv, helpHeader);
   MlirOptMainConfig config = MlirOptMainConfig::createFromCLOptions();
   config.preloadDialectsInContext(preloadDialectsInContext);
-
+  if (inputFilename == "-")
+    llvm::errs() << "(processing input from stdin now, hit ctrl-c/ctrl-d to "
+                    "interrupt)\n";
   // Set up the input file.
   std::string errorMessage;
   auto file = openInputFile(inputFilename, &errorMessage);
   if (!file) {
     llvm::errs() << errorMessage << "\n";
     return failure();
   }
 
   auto output = openOutputFile(outputFilename, &errorMessage);