cut detail helper is_atomic
cut detail helper is_atomic

Summary: Just use `is_instantiation_of` directly.

Reviewed By: dmm-fb

Differential Revision: D41062971

fbshipit-source-id: 9ddd73d28c81700b7f4aedff54178989b36f5bc6

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly