[flang] Set default form if unspecified at end of OPEN statement

[jeanPerier]

Summary

The standard specifies that the form must be set to a default at the
end of an open statement (C.7.4 point 2) where it was not specified.
Only in the case of a pre-connected unit can the form be deferred
until the first data transfer statement.

Set back the default form setting in OpenStatementState::EndIoStatement
that had been removed when deferring the pre-connected unit from setting
in 199a623ebf808a01e920ebd9904c99e633c33a1f.

Also return "UNDEFINED" instead of "UNKNOWN" to the FORM inquiry when the
form was not yet specified as defined in 12.10.2.12.

Related test: syntax 215a216.

[PeteSteinfeld]

I don't really understand the runtime code, but all builds, tests, and (as far as I can tell) looks good.

flang/runtime/io-stmt.cpp

if (!unit().isUnformatted) {
     unit().isUnformatted = isUnformatted_;
   }
   if (isUnformatted_ && *isUnformatted_ != *unit().isUnformatted) {
     if (wasExtant_) {
       SignalError("FORM= may not be changed on an open unit");
     }
     unit().isUnformatted = *isUnformatted_;
   }
+  if (!unit().isUnformatted) {
+    // Set default format (C.7.4 point 2).
+    unit().isUnformatted = unit().access != Access::Sequential;
+  }
   return ExternalIoStatementBase::EndIoStatement();
 }
 
 int CloseStatementState::EndIoStatement() {
   int result{ExternalIoStatementBase::EndIoStatement()};
   unit().CloseUnit(status_, *this);
   unit().DestroyClosed();
   return result;
str = unit().access == Access::Direct ||
         : "NO";
     break;
   case HashInquiryKeyword("ENCODING"):
     str = unit().isUnformatted.value_or(true) ? "UNDEFINED"
         : unit().isUTF8                       ? "UTF-8"
                                               : "ASCII";
     break;
   case HashInquiryKeyword("FORM"):
-    str = !unit().isUnformatted ? "UNKNOWN"
+    str = !unit().isUnformatted ? "UNDEFINED"
         : *unit().isUnformatted ? "UNFORMATTED"
                                 : "FORMATTED";
     break;
   case HashInquiryKeyword("FORMATTED"):
     str = !unit().isUnformatted ? "UNKNOWN"
         : *unit().isUnformatted ? "NO"
                                 : "YES";
     break;