[flang] Take into account SubprogramDetails in GetInterfaceSymbol

[jeanPerier]

Summary

When the ProcRef is Symbol is a SubprogramDetails, the interface is
the SubprogramDetails. Do not return nullptr.

flang/lib/Evaluate/call.cpp

 const Symbol *ProcedureDesignator::GetInterfaceSymbol() const {
   if (const Symbol * symbol{GetSymbol()}) {
     const Symbol &ultimate{symbol->GetUltimate()};
     if (const auto *proc{ultimate.detailsIf<semantics::ProcEntityDetails>()}) {
       return proc->interface().symbol();
     } else if (const auto *binding{
                    ultimate.detailsIf<semantics::ProcBindingDetails>()}) {
       return &binding->symbol();
+    } else if (ultimate.has<semantics::SubprogramDetails>()) {
+      return &ultimate;
     }
   }
   return nullptr;
 }
 
 bool ProcedureDesignator::IsElemental() const {
   if (const Symbol * interface{GetInterfaceSymbol()}) {
     return interface->attrs().test(semantics::Attr::ELEMENTAL);