Note [Concurrent read barrier on deRefWeak#]

In general the non-moving GC assumes that all pointers reachable from a
marked object are themselves marked (or in the mark queue). However,
weak pointers are an obvious exception to this rule. In particular,
deRefWeakPtr# allows the mutator to turn a weak reference into a strong
reference. This interacts badly with concurrent collection. For
instance, consider this program:

    f :: a -> b -> IO b
    f k v = do
        -- assume that k and v are the only references to the
        -- closures to which they refer.
        weak <- mkWeakPtr k v Nothing

        -- N.B. k is now technically dead since the only reference to it is
        -- weak, but we've not yet had a chance to tombstone the WeakPtr
        -- (which will happen in the course of major GC).
        performMajorGC
        -- Now we are running concurrently with the mark...

        Just x <- deRefWeak weak
        -- We have now introduced a reference to `v`, which will
        -- not be marked as the only reference to `v` when the snapshot was
        -- taken is via a WeakPtr.
        return x