[flang] Don't fold zero-length substring references

[klausler]

Summary

In some contexts the code looks cleaner if we fold zero-length substring
references into empty character constants, but those don't work in
variable definition contexts and can lead to bogus semantic error
messages.

flang/lib/Evaluate/fold-implementation.h

template <typename T> Expr<T> Folder<T>::Folding(Designator<T> &&designator) {
   if constexpr (T::category == TypeCategory::Character) {
     if (auto *substring{common::Unwrap<Substring>(designator.u)}) {
       if (std::optional<Expr<SomeCharacter>> folded{
               substring->Fold(context_)}) {
         if (const auto *specific{std::get_if<Expr<T>>(&folded->u)}) {
           return std::move(*specific);
         }
       }
-      if (auto length{ToInt64(Fold(context_, substring->LEN()))}) {
-        if (*length == 0) {
-          return Expr<T>{Constant<T>{Scalar<T>{}}};
-        }
-      }
+      // We used to fold zero-length substrings into zero-length
+      // constants here, but that led to problems in variable
+      // definition contexts.
     }
   } else if constexpr (T::category == TypeCategory::Real) {
     if (auto *zPart{std::get_if<ComplexPart>(&designator.u)}) {
       *zPart = FoldOperation(context_, std::move(*zPart));
       using ComplexT = Type<TypeCategory::Complex, T::kind>;
       if (auto zConst{Folder<ComplexT>{context_}.Folding(zPart->complex())}) {
         return Fold(context_,
             Expr<T>{ComplexComponent<T::kind>{