[flang] Don't drop format string for external child I/O

[klausler]

Summary

In user-defined derived type I/O to an external unit, don't
omit the format string from the constructor of ChildFormattedIoStatement.
And include any user IOMSG text in the crash message of the
parent, if it doesn't catch errors.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/runtime/format-implementation.h

       ++offset_;
     }
   } else if (edit.descriptor == 'D' && Capitalize(PeekNext()) == 'T') {
-    // DT'iotype'(v_list) user-defined derived type I/O
+    // DT['iotype'][(v_list)] user-defined derived type I/O
     edit.descriptor = DataEdit::DefinedDerivedType;
     ++offset_;
     if (auto quote{static_cast<char>(PeekNext())};

flang/runtime/io-api.cpp

                                         : "formatted input",
                false, DIR)
         ? &child->BeginIoStatement<ChildFormattedIoStatementState<DIR>>(
-              *child, sourceFile, sourceLine)
+              *child, format, formatLength, sourceFile, sourceLine)
         : nullptr;
   } else {
     if (!unit.isUnformatted.has_value()) {

flang/runtime/io-error.cpp

 
 void IoErrorHandler::Forward(
     int ioStatOrErrno, const char *msg, std::size_t length) {
-  SignalError(ioStatOrErrno);
-  if (ioStat_ != IostatOk && (flags_ & hasIoMsg)) {
+  if (ioStat_ != IostatOk && msg && (flags_ & hasIoMsg)) {
     ioMsg_ = SaveDefaultCharacter(msg, length, *this);
   }
+  if (ioStatOrErrno != IostatOk && msg) {
+    SignalError(ioStatOrErrno, "%.*s", static_cast<int>(length), msg);
+  } else {
+    SignalError(ioStatOrErrno);
+  }
 }
 
 void IoErrorHandler::SignalErrno() { SignalError(errno); }