Does standard C++11 guarantee that `volatile atomic<T>` has both semantics (volatile + atomic)?

Yes, it does.

Section 29.6.5, "Requirements for operations on atomic types"

Many operations are volatile-qualified. The “volatile as device register” semantics have not changed in the standard. This qualification means that volatility is preserved when applying these operations to volatile objects.

I checked working drafts 2008 through 2016, and the same text is in all of them. Therefore it should apply C++11, C++14, and C++17.