[flang] Allow for deferred-length character in EstablishDescriptor

[klausler]

Summary

When the runtime is initializing an instance of a derived type,
don't crash if an allocatable character component has deferred length.

[jeanPerier]

LGTM, thanks

flang/runtime/type-info.h

     Explicit = 2,
     LenParameter = 3
   };
-
+  Genre genre() const { return genre_; }
   std::optional<TypeParameterValue> GetValue(const Descriptor *) const;
 
 private:

flang/runtime/type-info.cpp

     const Descriptor &container, Terminator &terminator) const {
   TypeCategory cat{category()};
   if (cat == TypeCategory::Character) {
-    auto length{characterLen_.GetValue(&container)};
-    RUNTIME_CHECK(terminator, length.has_value());
-    descriptor.Establish(kind_, *length / kind_, nullptr, rank_);
+    std::size_t lengthInChars{0};
+    if (auto length{characterLen_.GetValue(&container)}) {
+      lengthInChars = static_cast<std::size_t>(*length / kind_);
+    } else {
+      RUNTIME_CHECK(
+          terminator, characterLen_.genre() == Value::Genre::Deferred);
+    }
+    descriptor.Establish(kind_, lengthInChars, nullptr, rank_);
   } else if (cat == TypeCategory::Derived) {
     const DerivedType *type{derivedType()};
     RUNTIME_CHECK(terminator, type != nullptr);