parallelize get_type_usages
parallelize get_type_usages

Summary: Use a `ConcurrentMap` and `walk::parallel::opcodes` to speed up `get_type_usages`

Reviewed By: thezhangwei

Differential Revision: D19305432

fbshipit-source-id: bf4d7e3f5fbdccd5695d5b0b0937c4defd4f0762

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