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.
One of the common uses of std::nullopt is in one of the constructors
for ValueRange. This patch takes care of the migration where we need
ValueRange() to facilitate perfect forwarding. Note that {} would be
ambiguous for perfecting forwarding to work.
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