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

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.

Differential Revision: https://reviews.llvm.org/D117903

flang/runtime/format-implementation.h

@@ -353,7 +353,7 @@ DataEdit FormatControl<CONTEXT>::GetNextDataEdit(
       ++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

@@ -215,7 +215,7 @@ Cookie BeginExternalFormattedIO(const char *format, std::size_t formatLength,
                                         : "formatted input",
                false, DIR)
         ? &child->BeginIoStatement<ChildFormattedIoStatementState<DIR>>(
-              *child, sourceFile, sourceLine)
+              *child, format, formatLength, sourceFile, sourceLine)
         : nullptr;
   } else {
     IoErrorHandler handler{terminator};

flang/runtime/io-error.cpp

@@ -59,10 +59,14 @@ void IoErrorHandler::SignalError(int iostatOrErrno) {
 
 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); }