MPMCQueue: Fix anonymous union of T and Atom<T> treatment
MPMCQueue: Fix anonymous union of T and Atom<T> treatment

Summary: The previous code issued stores/loads to the non-atomic anonymous union members from partially specialized Dynamic=true code. This breaks when Atom is a class that doesn't behave like a std::atomic.

Reviewed By: magedm

Differential Revision: D10502987

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