[flang][runtime] Allow record advancement in child I/O via '/' control edit descriptor

[klausler]

Summary

My earlier misreading of the Fortran standards had convinced me that child I/O

• meaning the use of user-defined subroutines via generic interfaces to implement

data transfer statements -- was not allowed to advance the current record in the
ultimate unit of the original (non-child parent) data transfer statement.
This turns out to be wrong, so forward AdvanceRecord() from ChildFormattedIoStatement<>
to its parent I/O statement rather than implementing it as a no-op.

[clementval]

LGTM

flang/runtime/io-stmt.cpp

 }
 
 template <Direction DIR, typename CHAR>
-bool ChildFormattedIoStatementState<DIR, CHAR>::AdvanceRecord(int) {
-  return false; // no can do in a child I/O
+bool ChildFormattedIoStatementState<DIR, CHAR>::AdvanceRecord(int n) {
+  return this->child().parent().AdvanceRecord(n);
 }
 
 template <Direction DIR>