ForLoopHeader, instead assuming that all ranges are closed and that the
"last" property is already property calculated.
This means the HeaderInfo.last for arrays, "indices" and "until"
progressions are decremented from the bound. This simplifies the loop
construction and moves the work to the handlers. This will also simplify
the handling of "reversed()" in the future since we don't need to
consider the case where the lower bound is open.
Mark Punzalan
301ac90770