move input shared_ptr in atomic_shared_ptr constructor
move input shared_ptr in atomic_shared_ptr constructor

Summary: By moving the input shared_ptr instead of copying it, we avoid doing an unnecessary ref count increment + decrement.

Reviewed By: djwatson, yfeldblum

Differential Revision: D5298467

fbshipit-source-id: b9f0b6999278609417bb4dc062030ca2388ba20a

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