Enable #nullable for disposable utils.

[bartdesmet]

One more bit of #nullable support. All of the helpers for IDisposable operations are rather gnarly given their excessive use of ref because of the underlying Volatile and Interlocked operations. Luckily we can flow nullable analysis through this using various attributes. In particular:

• read-only uses of ref (which could be in if the underlying APIs were to use in) can be annotated to ensure null-invariance;
• assignments through ref can be annotated to inherit their null-ness from the value being assigned.