[flang][openacc] Add proper TODO for reduction lowering

[clementval]

Summary

Reduction clause on loop or compute constructs
is not currently lowered but no TODO message was issued.
Add a proper TODO so the user is informed.

[clementval]

Move the TODO for compute construct

flang/lib/Lower/OpenACC.cpp

if (const auto *gangClause =
           tileOperands.push_back(tileStar);
         }
       }
     } else if (const auto *privateClause =
                    std::get_if<Fortran::parser::AccClause::Private>(
                        &clause.u)) {
       genObjectList(privateClause->v, converter, semanticsContext, stmtCtx,
                     privateOperands);
-    }
+    } else if (std::get_if<Fortran::parser::AccClause::Reduction>(&clause.u)) {
       // Reduction clause is left out for the moment as the clause will probably
       // end up having its own operation.
+      TODO(clauseLocation, "OpenACC compute construct reduction lowering");
+    }
   }
 
   // Prepare the operand segment size attribute and the operands value range.
   llvm::SmallVector<mlir::Value> operands;
   llvm::SmallVector<int32_t> operandSegments;
   addOperand(operands, operandSegments, gangNum);
   addOperand(operands, operandSegments, gangStatic);
   addOperand(operands, operandSegments, workerNum);
if (const auto *asyncClause =
                        &clause.u)) {
       genObjectList(privateClause->v, converter, semanticsContext, stmtCtx,
                     privateOperands);
     } else if (const auto *firstprivateClause =
                    std::get_if<Fortran::parser::AccClause::Firstprivate>(
                        &clause.u)) {
       genObjectList(firstprivateClause->v, converter, semanticsContext, stmtCtx,
                     firstprivateOperands);
+    } else if (std::get_if<Fortran::parser::AccClause::Reduction>(&clause.u)) {
+      TODO(clauseLocation, "compute construct reduction clause lowering");
     }
   }
 
   // Prepare the operand segment size attribute and the operands value range.
   llvm::SmallVector<mlir::Value, 8> operands;
   llvm::SmallVector<int32_t, 8> operandSegments;
   addOperand(operands, operandSegments, async);
   addOperands(operands, operandSegments, waitOperands);