[flang] Ensure "dtv" args are passed by descriptor when polymorphic

[klausler]

Summary

The predicate used to determine whether the initial "dtv" argument
to a user-defined derived type I/O procedure specified by a generic
interface that is not in the derived type definition is wrong.
The "dtv" argument must be passed via a descriptor whenever it is
polymorphic.

[clementval]

LGTM

flang/lib/Semantics/runtime-type-info.cpp

if (isAssignment) {
           if (int rank{evaluate::GetRank(typeAndShape.shape())}; rank > 0) {
             argThatMightBeDescriptor = 1;
             which = IntExpr<1>(ToInt64(which).value() + rank);
           }
         }
       }
     } else { // user defined derived type I/O
       CHECK(proc->dummyArguments.size() >= 4);
+      const auto *ddo{std::get_if<evaluate::characteristics::DummyDataObject>(
+          &proc->dummyArguments[0].u)};
+      if (!ddo) {
+        return;
+      }
       if (derivedTypeSpec &&
-          !std::get<evaluate::characteristics::DummyDataObject>(
-              proc->dummyArguments[0].u)
-               .type.type()
-               .IsTkCompatibleWith(evaluate::DynamicType{*derivedTypeSpec})) {
+          !ddo->type.type().IsTkCompatibleWith(
+              evaluate::DynamicType{*derivedTypeSpec})) {
         // Defined I/O specific procedure is not for this derived type.
         return;
       }
-      if (binding) {
+      if (ddo->type.type().IsPolymorphic()) {
         isArgDescriptorSet |= 1;
       }
       switch (io.value()) {
       case GenericKind::DefinedIo::ReadFormatted:
         which = readFormattedEnum_;
         break;
       case GenericKind::DefinedIo::ReadUnformatted:
         which = readUnformattedEnum_;