Make atomic functions return scope, not ref return

[dkorpel]

I made a mistake in #3480 by adding just return without scope. That would suggest ref T val would return a T* like &val, but they return a T. This trips up SharedRegion in std.allocator after applying dlang/dmd#12812 because of this:

localCurrent = atomicLoad(this._current);

Which would needlessly turn localCurrent into a scope pointer since atomicLoad's signature suggests it's essentially returning &this which it isn't.

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