[flang] Prevent bogus runtime I/O error message

[klausler]

Summary

The runtime was requiring that STATUS='OLD' be explicitly specified
on an OPEN statement for a connected unit. There error should issue
only if a STATUS= other than 'OLD' is specified; an OPEN with no
STATUS= specifier is okay.

flang/runtime/io-stmt.h

public:
   void set_status(OpenStatus status) { status_ = status; } // STATUS=
   void set_path(const char *, std::size_t, int kind); // FILE=
   void set_position(Position position) { position_ = position; } // POSITION=
   void set_action(Action action) { action_ = action; } // ACTION=
   int EndIoStatement();
 
 private:
   bool wasExtant_;
-  OpenStatus status_{OpenStatus::Unknown};
+  std::optional<OpenStatus> status_;
   Position position_{Position::AsIs};
   std::optional<Action> action_;
   OwningPtr<char> path_;
   std::size_t pathLength_;
 };
 
 class CloseStatementState : public ExternalIoStatementBase {
 public:

flang/runtime/io-stmt.cpp

if (kind != 1) { // TODO
     Crash("OPEN: FILE= with unimplemented: CHARACTER(KIND=%d)", kind);
   }
   std::size_t bytes{length * kind}; // TODO: UTF-8 encoding of Unicode path
   path_ = SaveDefaultCharacter(path, bytes, *this);
   pathLength_ = length;
 }
 
 int OpenStatementState::EndIoStatement() {
-  if (wasExtant_ && status_ != OpenStatus::Old) {
-    SignalError("OPEN statement for connected unit must have STATUS='OLD'");
+  if (wasExtant_ && status_ && *status_ != OpenStatus::Old) {
+    SignalError("OPEN statement for connected unit may not have STATUS= other "
+                "than 'OLD'");
   }
-  unit().OpenUnit(
-      status_, action_, position_, std::move(path_), pathLength_, *this);
+  unit().OpenUnit(status_.value_or(OpenStatus::Unknown), action_, position_,
+      std::move(path_), pathLength_, *this);
   return ExternalIoStatementBase::EndIoStatement();
 }
 
 int CloseStatementState::EndIoStatement() {
   int result{ExternalIoStatementBase::EndIoStatement()};
   unit().CloseUnit(status_, *this);
   unit().DestroyClosed();
   return result;