[flang][runtime] Initialize LHS temporary in AssignTemporary.

[vzakhari]

Summary

If LHS is of derived type that needs initialization, then it must be
initialized before doing the assignment. Otherwise, the assignment
might behave incorrectly based on uninitialized components that are
descriptors themselves.

[vzakhari]

clang-format

[jeanPerier]

Looks good, thanks

flang/runtime/assign.cpp

void RTNAME(Assign)(Descriptor &to, const Descriptor &from,
   // defined assignment apart from components.
   Assign(to, from, terminator,
       MaybeReallocate | NeedFinalization | ComponentCanBeDefinedAssignment);
 }
 
 void RTNAME(AssignTemporary)(Descriptor &to, const Descriptor &from,
     const char *sourceFile, int sourceLine) {
   Terminator terminator{sourceFile, sourceLine};
+  // Initialize the "to" if it is of derived type that needs initialization.
+  if (const DescriptorAddendum * addendum{to.Addendum()}) {
+    if (const auto *derived{addendum->derivedType()}) {
+      if (!derived->noInitializationNeeded()) {
+        if (ReturnError(terminator, Initialize(to, *derived, terminator)) !=
+            StatOk) {
+          return;
+        }
+      }
+    }
+  }
   Assign(to, from, terminator, PolymorphicLHS);
 }
 
 void RTNAME(AssignExplicitLengthCharacter)(Descriptor &to,
     const Descriptor &from, const char *sourceFile, int sourceLine) {
   Terminator terminator{sourceFile, sourceLine};
   Assign(to, from, terminator,
       MaybeReallocate | NeedFinalization | ComponentCanBeDefinedAssignment |