[flang][hlfir] Add TODO for polymorphic vector subscripted entities

[jeanPerier]

Summary

I removed the barriers a bit fast. Some mold is needed on
hlfir.elemental for polymorphic vector subscripted designator (and for
parenthesized polymorphic) so that a temporary can be later created.

The parenthesized array case may also just used asExpr and that
could later use AssignTemporary to deal with this. But the vector
subscripted designator case will need to use some new runtime
to get some mold allocation done for the temp.

Add TODOs in the meantime.

[clementval]

LGTM

[vzakhari]

Thank you, Jean!

flang/lib/Lower/ConvertExprToHLFIR.cpp

private:
   /// be turned into an hlfir.elemental value, unless the caller of this
   /// utility requested to get the hlfir.elemental_addr instead of lowering
   /// the designator to an mlir::Value.
   mlir::Type createVectorSubscriptElementAddrOp(
       PartInfo &partInfo, mlir::Type baseType,
       llvm::ArrayRef<mlir::Value> resultExtents) {
     fir::FirOpBuilder &builder = getBuilder();
     mlir::Value shape = builder.genShape(loc, resultExtents);
+    // For polymorphic entities, it will be needed to add a mold on the
+    // hlfir.elemental_addr/hlfir.elemental so that we are able to create
+    // temporary storage for it.
+    if (partInfo.base && partInfo.base->isPolymorphic())
+      TODO(loc, "vector subscripted polymorphic entity in HLFIR");
     // The type parameters to be added on the hlfir.elemental_addr are the ones
     // of the whole designator (not the ones of the vector subscripted part).
     // These are not yet known and will be added when finalizing the designator
     // lowering.
     auto elementalAddrOp = builder.create<hlfir::ElementalAddrOp>(loc, shape);
     setVectorSubscriptElementAddrOp(elementalAddrOp);
     builder.setInsertionPointToEnd(&elementalAddrOp.getBody().front());
     mlir::Region::BlockArgListType indices = elementalAddrOp.getIndices();
if constexpr (R::category == Fortran::common::TypeCategory::Character) {
       unaryOp.genResultTypeParams(loc, builder, left, typeParams);
     }
     if (rank == 0)
       return unaryOp.gen(loc, builder, op.derived(), left);
 
     // Elemental expression.
     mlir::Type elementType;
     if constexpr (R::category == Fortran::common::TypeCategory::Derived) {
+      // TODO: need to pass a mold to hlfir.elemental for polymorphic arrays
+      // if using hlfir.elemental here so that it can get the dynamic type
+      // info.
+      if (left.isPolymorphic())
+        TODO(loc, "parenthesized polymorphic arrays in HLFIR");
       elementType = Fortran::lower::translateDerivedTypeToFIRType(
           getConverter(), op.derived().GetType().GetDerivedTypeSpec());
     } else {
       elementType =
           Fortran::lower::getFIRType(builder.getContext(), R::category, R::kind,
                                      /*params=*/std::nullopt);
     }
     mlir::Value shape = hlfir::genShape(loc, builder, left);