If you allocate a StringData on the stack, and it escapes,
you're in trouble. Make the destructor assert by default,
and add a StackStringData which does the appropriate
refCounting and checking.
This change is mostly for FB internal organizational reasons.
Building is not effected beyond the fact that the target now
lands in hphp/hhvm/hhvm rather than src/hhvm/hhvm.