[mlir] mlir-opt: print a newline after the top-level module

A printer refactoring removed automatic newline printing in the printer
of a ModuleOp. As a consequence, mlir-opt no longer printed a newline
after the closing brace of a module, which made it hard to distinguish
when used from command line. Print the newline character explicitly in
mlir-opt.

Author: ftynse

mlir/lib/Support/MlirOptMain.cpp

@@ -59,6 +59,7 @@ static LogicalResult performActions(raw_ostream &os, bool verifyDiagnostics,
 
   // Print the output.
   module->print(os);
+  os << '\n';
   return success();
 }