[flang] Replace crash and improve a semantics TODO message

[klausler]

Summary

The derived type information table construction code had a
crash whose root cause was replacing an expression with one
of its operands -- the deletion of the LHS of that assignment
led to the RHS being invalidated before it could be read.
Fix by cloning the RHS. Also update a TODO message to the
new "_todo_en_US" message class and add a comment about how
it should be resolved.

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

evaluate::StructureConstructor GetValue(
     }
     if (expr) {
       if (parameters) {
         if (const Symbol * lenParam{evaluate::ExtractBareLenParameter(*expr)}) {
           return PackageIntValue(
               lenParameterEnum_, FindLenParameterIndex(*parameters, *lenParam));
         }
       }
+      // TODO: Replace a specification expression requiring actual operations
+      // with a reference to a new anonymous LEN type parameter whose default
+      // value captures the expression.  This replacement must take place when
+      // the type is declared so that the new LEN type parameters appear in
+      // all instantiations and structure constructors.
       context_.Say(location_,
-          "Specification expression '%s' is neither constant nor a length "
-          "type parameter"_err_en_US,
+          "derived type specification expression '%s' that is neither constant nor a length type parameter"_todo_en_US,
           expr->AsFortran());
     }
     return PackageIntValue(deferredEnum_);
   }
 
   SemanticsContext &context_;
   RuntimeDerivedTypeTables &tables_;
   std::map<const Symbol *, SymbolVector> orderedTypeParameters_;
if (const semantics::DerivedTypeSpec *
     len = Fold(foldingContext, std::move(len));
   }
   if (dyType.category() == TypeCategory::Character && len) {
     // Ignore IDIM(x) (represented as MAX(0, x))
     if (const auto *clamped{evaluate::UnwrapExpr<
             evaluate::Extremum<evaluate::SubscriptInteger>>(*len)}) {
       if (clamped->ordering == evaluate::Ordering::Greater &&
           clamped->left() == evaluate::Expr<evaluate::SubscriptInteger>{0}) {
-        len = clamped->right();
+        len = common::Clone(clamped->right());
       }
     }
     AddValue(values, componentSchema_, "characterlen"s,
         evaluate::AsGenericExpr(GetValue(len, parameters)));
   } else {
     AddValue(values, componentSchema_, "characterlen"s,
         PackageIntValueExpr(deferredEnum_));
   }