[flang][runtime] Return the right mutable modes from INQUIRE in child I/O

[klausler]

Summary

During child I/O -- a call to a user-defined subroutine with a generic interface
to handle read and write operations on a specific derived type -- ensure that
any INQUIRE statement probing mutable modes like BLANK= will observe changes
that have been made to those modes via control edit descriptors like BN
while processing the original I/O statement's format.

[clementval]

LGTM

flang/runtime/io-stmt.cpp

     ExternalFileUnit &unit, const char *sourceFile, int sourceLine)
     : IoStatementBase{sourceFile, sourceLine}, unit_{unit} {}
 
-MutableModes &ExternalIoStatementBase::mutableModes() { return unit_.modes; }
+MutableModes &ExternalIoStatementBase::mutableModes() {
+  if (const ChildIo * child{unit_.GetChildIo()}) {
+    return child->parent().mutableModes();
+  }
+  return unit_.modes;
+}
 
 ConnectionState &ExternalIoStatementBase::GetConnectionState() { return unit_; }
 

   case HashInquiryKeyword("BLANK"):
     str = !unit().IsConnected() || unit().isUnformatted.value_or(true)
         ? "UNDEFINED"
-        : unit().modes.editingFlags & blankZero ? "ZERO"
+        : mutableModes().editingFlags & blankZero ? "ZERO"
                                                   : "NULL";
     break;
   case HashInquiryKeyword("CARRIAGECONTROL"):
     str = "LIST";

   case HashInquiryKeyword("DECIMAL"):
     str = !unit().IsConnected() || unit().isUnformatted.value_or(true)
         ? "UNDEFINED"
-        : unit().modes.editingFlags & decimalComma ? "COMMA"
+        : mutableModes().editingFlags & decimalComma ? "COMMA"
                                                      : "POINT";
     break;
   case HashInquiryKeyword("DELIM"):
     if (!unit().IsConnected() || unit().isUnformatted.value_or(true)) {
       str = "UNDEFINED";
     } else {
-      switch (unit().modes.delim) {
+      switch (mutableModes().delim) {
       case '\'':
         str = "APOSTROPHE";
         break;

   case HashInquiryKeyword("PAD"):
     str = !unit().IsConnected() || unit().isUnformatted.value_or(true)
         ? "UNDEFINED"
-        : unit().modes.pad ? "YES"
+        : mutableModes().pad ? "YES"
                              : "NO";
     break;
   case HashInquiryKeyword("POSITION"):
     if (!unit().IsConnected() || unit().access == Access::Direct) {

     if (!unit().IsConnected() || unit().isUnformatted.value_or(true)) {
       str = "UNDEFINED";
     } else {
-      switch (unit().modes.round) {
+      switch (mutableModes().round) {
       case decimal::FortranRounding::RoundNearest:
         str = "NEAREST";
         break;

   case HashInquiryKeyword("SIGN"):
     str = !unit().IsConnected() || unit().isUnformatted.value_or(true)
         ? "UNDEFINED"
-        : unit().modes.editingFlags & signPlus ? "PLUS"
+        : mutableModes().editingFlags & signPlus ? "PLUS"
                                                  : "SUPPRESS";
     break;
   case HashInquiryKeyword("STREAM"):
     str = !unit().IsConnected()           ? "UNKNOWN"