[flang][runtime][NFC] Add a comment to intrinsic assignment (llvm#153260)

Add a comment explaining why intrinsic derived type assignment
unconditionally deallocates all allocated allocatable subobject
components of the left-hand side variable, so that I won't forget the
reasoning here the next time this comes into question.

Author: klausler

flang-rt/lib/runtime/assign.cpp

@@ -369,6 +369,9 @@ RT_API_ATTRS int AssignTicket::Begin(WorkQueue &workQueue) {
         return status;
       }
     } else if (!toDerived_->noDestructionNeeded()) {
+      // F'2023 9.7.3.2 p7: "When an intrinsic assignment statement (10.2.1.3)
+      // is executed, any noncoarray allocated allocatable subobject of the
+      // variable is deallocated before the assignment takes place."
       if (int status{
               workQueue.BeginDestroy(to_, *toDerived_, /*finalize=*/false)};
           status != StatOk && status != StatContinue) {