[flang] runtime: fix WRITE after BACKSPACE on variable-length file

[klausler]

Summary

BACKSPACE leaves "recordLength" set, which is fine for a later READ,
but it causes a later WRITE to fail due to a misinterpretation of the
knowledge of the record length as indication of a fixed-length record
file (RECL=). Fix.

[jeanPerier]

LGTM

flang/runtime/unit.cpp

if (elementBytes > 1) {
     }
   }
 }
 
 bool ExternalFileUnit::Emit(const char *data, std::size_t bytes,
     std::size_t elementBytes, IoErrorHandler &handler) {
   auto furthestAfter{std::max(furthestPositionInRecord,
       positionInRecord + static_cast<std::int64_t>(bytes))};
-  if (furthestAfter > recordLength.value_or(furthestAfter)) {
+  if (recordLength) {
+    // It is possible for recordLength to have a value now for a
+    // variable-length output record if the previous operation
+    // was a BACKSPACE.
+    if (!isFixedRecordLength) {
+      recordLength.reset();
+    } else if (furthestAfter > *recordLength) {
       handler.SignalError(IostatRecordWriteOverrun,
-        "Attempt to write %zd bytes to position %jd in a fixed-size record of "
-        "%jd bytes",
+          "Attempt to write %zd bytes to position %jd in a fixed-size record "
+          "of %jd bytes",
           bytes, static_cast<std::intmax_t>(positionInRecord),
           static_cast<std::intmax_t>(*recordLength));
       return false;
     }
+  }
   WriteFrame(frameOffsetInFile_, recordOffsetInFrame_ + furthestAfter, handler);
   if (positionInRecord > furthestPositionInRecord) {
     std::memset(Frame() + recordOffsetInFrame_ + furthestPositionInRecord, ' ',
         positionInRecord - furthestPositionInRecord);
   }
   char *to{Frame() + recordOffsetInFrame_ + positionInRecord};
   std::memcpy(to, data, bytes);
   if (swapEndianness_) {