[flang] Add TODO instead of crashing on assert

[clementval]

Summary

Current code are crashing on the assert assert(seqTy && "must be an array");.

Add a TODO instead until the support is in.

[jeanPerier]

LGTM

flang/lib/Lower/ConvertExpr.cpp

genIterSpace(mlir::Type resultType) {
     // We're ready to lower the body (an assignment statement) for this context
     // of loop nests at this point.
     return {iters, afterLoopNest};
   }
 
   fir::ArrayLoadOp
   createAndLoadSomeArrayTemp(mlir::Type type,
                              llvm::ArrayRef<mlir::Value> shape) {
+    mlir::Location loc = getLoc();
+    if (fir::isPolymorphicType(type))
+      TODO(loc, "polymorphic array temporary");
     if (ccLoadDest)
       return (*ccLoadDest)(shape);
     auto seqTy = type.dyn_cast<fir::SequenceType>();
     assert(seqTy && "must be an array");
-    mlir::Location loc = getLoc();
     // TODO: Need to thread the LEN parameters here. For character, they may
     // differ from the operands length (e.g concatenation). So the array loads
     // type parameters are not enough.
     if (auto charTy = seqTy.getEleTy().dyn_cast<fir::CharacterType>())
       if (charTy.hasDynamicLen())
         TODO(loc, "character array expression temp with dynamic length");
     if (auto recTy = seqTy.getEleTy().dyn_cast<fir::RecordType>())
       if (recTy.getNumLenParams() > 0)