Trace reachability stacks
Trace reachability stacks

Summary:
It's convenient to see the whole stack trace to figure out why any
given item is reachable.  Also, record the reason why annos are kept.

Reviewed By: dariorussi

Differential Revision: D4539952

fbshipit-source-id: 49683eb98e14a1f743bc33e6afc6ff0034ea4824

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