[flang] Add TODO when trying to do a polymorphic temp in getTempExtAddr

[jeanPerier]

LGTM

flang/lib/Lower/ConvertExpr.cpp

return genExtValue(expr).match(
           if (fir::isa_ref_type(type))
             value = builder.create<fir::LoadOp>(loc, value);
           mlir::Value temp = builder.createTemporary(loc, value.getType());
           builder.create<fir::StoreOp>(loc, value, temp);
           return temp;
         },
         [&](const fir::BoxValue &x) -> ExtValue {
           // Derived type scalar that may be polymorphic.
+          if (fir::isPolymorphicType(fir::getBase(x).getType()))
+            TODO(loc, "polymorphic array temporary");
           assert(!x.hasRank() && x.isDerived());
           if (x.isDerivedWithLenParameters())
             fir::emitFatalError(
                 loc, "making temps for derived type with length parameters");
           // TODO: polymorphic aspects should be kept but for now the temp
           // created always has the declared type.
           mlir::Value var =
               fir::getBase(fir::factory::readBoxValue(builder, loc, x));