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

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 199a623.

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.

Differential Revision: https://reviews.llvm.org/D108990

Author: jeanPerier

flang/runtime/io-stmt.cpp

@@ -243,6 +243,10 @@ int OpenStatementState::EndIoStatement() {
     }
     unit().isUnformatted = *isUnformatted_;
   }
+  if (!unit().isUnformatted) {
+    // Set default format (C.7.4 point 2).
+    unit().isUnformatted = unit().access != Access::Sequential;
+  }
   return ExternalIoStatementBase::EndIoStatement();
 }
 
@@ -969,7 +973,7 @@ bool InquireUnitState::Inquire(
                                               : "ASCII";
     break;
   case HashInquiryKeyword("FORM"):
-    str = !unit().isUnformatted ? "UNKNOWN"
+    str = !unit().isUnformatted ? "UNDEFINED"
         : *unit().isUnformatted ? "UNFORMATTED"
                                 : "FORMATTED";
     break;