[Attributor] Add "free"-based heap2stack deduction

Summary:
If there is a unique free of the allocated that has to be reached from
the malloc, we can apply the heap-2-stack transformation even if the
pointer escapes.

Reviewers: hfinkel, sstefan1, uenoku

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68958