Extend AllocBoxToStack to handle apply

AllocBoxToStack analyzes the uses of boxes and promotes them to stack if
it is safe to do so. Currently the analysis is limited to only a few known
users including partial_apply.

With this change, the pass also analyzes apply users, where the callee
is a local private function.
The analysis is recursive and bound by a threshold.

Fixes rdar://59070139

Author: meg-gupta