This diff addresses what we called "step 1" in the task: simply ensure that any C++ exceptions that escape a destructor get rethrown and can continue to propagate naturally. The exception is remembered on the thread, and rethrown when we check for surprises later. If multiple destructors let C++ exceptions escape the last one to escape will be the one rethrown at the next surprise check.
This also ensures that C++ exceptions prevent more PHP code from running, by omitting calls to __destruct methods as we unwind the stack.
Finally, this also enables surprise checks for OnFunctionExit unless we're unwinding, in which case surprises remain unchecked so they can propagate later.
This is different than Zend's behavior, where destructors do run as fatals unwind.
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.
Mike Magruder