[flang] Define new runtime error IOSTAT values (I/O runtime work part 4)

[klausler]

Summary

Add more IOSTAT= values for errors that can arise in external I/O.

flang/runtime/iostat.h

enum Iostat {
 
   // The remaining error codes are not exported.
   IostatGenericError = 1001, // see IOMSG= for details
   IostatRecordWriteOverrun,
   IostatRecordReadOverrun,
   IostatInternalWriteOverrun,
   IostatErrorInFormat,
   IostatErrorInKeyword,
+  IostatEndfileNonSequential,
+  IostatEndfileUnwritable,
+  IostatOpenBadRecl,
+  IostatOpenUnknownSize,
+  IostatOpenBadAppend,
+  IostatWriteToReadOnly,
+  IostatReadFromWriteOnly,
+  IostatBackspaceNonSequential,
+  IostatBackspaceAtFirstRecord,
+  IostatRewindNonSequential,
 };
 
 const char *IostatErrorString(int);
 
 } // namespace Fortran::runtime::io
 #endif // FORTRAN_RUNTIME_IOSTAT_H_

flang/runtime/iostat.cpp

const char *IostatErrorString(int iostat) {
   case IostatRecordReadOverrun:
     return "Excessive input from fixed-size record";
   case IostatInternalWriteOverrun:
     return "Internal write overran available records";
   case IostatErrorInFormat:
     return "Invalid FORMAT";
   case IostatErrorInKeyword:
     return "Bad keyword argument value";
+  case IostatEndfileNonSequential:
+    return "ENDFILE on non-sequential file";
+  case IostatEndfileUnwritable:
+    return "ENDFILE on read-only file";
+  case IostatOpenBadRecl:
+    return "OPEN with bad RECL= value";
+  case IostatOpenUnknownSize:
+    return "OPEN of file of unknown size";
+  case IostatOpenBadAppend:
+    return "OPEN(POSITION='APPEND') of unpositionable file";
+  case IostatWriteToReadOnly:
+    return "Attempted output to read-only file";
+  case IostatReadFromWriteOnly:
+    return "Attempted input from write-only file";
+  case IostatBackspaceNonSequential:
+    return "BACKSPACE on non-sequential file";
+  case IostatBackspaceAtFirstRecord:
+    return "BACKSPACE at first record";
+  case IostatRewindNonSequential:
+    return "REWIND on non-sequential file";
   default:
     return nullptr;
   }
 }
 
 } // namespace Fortran::runtime::io