[flang][runtime] Block bad left tabbing in child I/O (#158758)

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 llvm/llvm-project#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_;
 }