Work around a bug in MSVC name lookup within templated friend contexts
Work around a bug in MSVC name lookup within templated friend contexts

Summary: MSVC has a bug that causes it to think that `once_flag` in this context resolves to the `using` statement further up, rather than `detail::once_flag`.

Reviewed By: yfeldblum

Differential Revision: D6376501

fbshipit-source-id: 481dbd75ed21f3d519bd920258fa743f314668ad

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