ArrayRef has a constructor that accepts std::nullopt. This
constructor dates back to the days when we still had llvm::Optional.
Since the use of std::nullopt outside the context of std::optional is
kind of abuse and not intuitive to new comers, I would like to move
away from the constructor and eventually remove it.
This patch migrates away from std::nullopt in favor of ArrayRef<T>()
where we use perfect forwarding. Note that {} would be ambiguous for
perfect forwarding to work.
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