folly: Alias ConcurrentHashMapSIMD to use SIMDTable backend
folly: Alias ConcurrentHashMapSIMD to use SIMDTable backend

Summary: The Impl param is last so its a pain to switch between BucketTable and SIMDTable in the common case where only KeyType and ValueType are specified. This diff enables ConcurrentHashMapSIMD<key, value>.

Reviewed By: djwatson

Differential Revision: D14916671

fbshipit-source-id: 980518179b87316ae344cbf96be4e6bcc032424e

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