Use folly namespace for make_unique calls when necessary
Use folly namespace for make_unique calls when necessary

Summary: Clang fails to disambiguate between std::make_unique and folly::make_unique when make_unique isn't qualified with a namespace. This diff fixes this problem by explicitly specifying the folly namespace.

Test Plan: unit tests

Reviewed By: @​mshneer

Differential Revision: D2046286
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