Pass custom comparator for dex output by reference
Pass custom comparator for dex output by reference

Summary:
std::sort copies the comparator around.  They're intended to be
stateless, but this one contains an internal map.  If we pass it by
reference_wrapper instead, then it's fast.

Reviewed By: onomatopoeia, fryfrog

Differential Revision: D3650070

fbshipit-source-id: 23b50d9918620e80596426f400d4b7779db4f58e

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