[flang] Add a proper TODO for polymorphic array lowering with vector subscript

[clementval]

Summary

Creation of polymorphic array temporary cannot be done inlined.
Add a TODO so the current code exit in a clean way when lowering
reach it. A solution involving the runtime will be put in place.

Depends on D143490

[jeanPerier]

LGTM

flang/lib/Lower/ConvertExpr.cpp

static mlir::Type adjustedArraySubtype(mlir::Type ty,
     mlir::Type pathTy = fir::applyPathToType(ty, indices);
     assert(pathTy && "indices failed to apply to type");
     return adjustedArrayElementType(pathTy);
   }
 
   /// Lower rhs of an array expression.
   ExtValue lowerArrayExpression(const Fortran::lower::SomeExpr &exp) {
     mlir::Type resTy = converter.genType(exp);
+
+    if (fir::isPolymorphicType(resTy) &&
+        Fortran::evaluate::HasVectorSubscript(exp))
+      TODO(getLoc(),
+           "polymorphic array expression lowering with vector subscript");
+
     return std::visit(
         [&](const auto &e) { return lowerArrayExpression(genarr(e), resTy); },
         exp.u);
   }
   ExtValue lowerArrayExpression(const ExtValue &exv) {
     assert(!explicitSpace);
     mlir::Type resTy = fir::unwrapPassByRefType(fir::getBase(exv).getType());
     return lowerArrayExpression(genarr(exv), resTy);