message: Forward-declare LogicalResult as struct rather than class
  Windows builds are broken by a class/struct mismatch between a
  forward-declaration of LogicalResult and its definition.
diff: @@ -30,7 +30,7 @@ namespace mlir {
  class DialectConversion;
  class FuncOp;
  class LLVMTypeConverter;
  -class LogicalResult;
  +struct LogicalResult;
  class MLIRContext;
  class ModuleOp;
  class ModulePassBase;
  