[flang][runtime] Block bad left tabbing in child I/O

Child I/O subroutines are not supposed to use T or TL
control edit descriptors in formats to move the position
in the current record to a point before where it stood at
the time of their calls (F'2023 12.6.4.8.3 paragraph 18),
but we should also guard against attempts to do so, using the
same means used to prevent such attempts in non-advancing I/O.

Fixes #158723.

Author: klausler

flang-rt/lib/runtime/unit.cpp

@@ -827,6 +827,7 @@ ChildIo &ExternalFileUnit::PushChildIo(IoStatementState &parent) {
   Terminator &terminator{parent.GetIoErrorHandler()};
   OwningPtr<ChildIo> next{New<ChildIo>{terminator}(parent, std::move(current))};
   child_.reset(next.release());
+  leftTabLimit = positionInRecord;
   return *child_;
 }