llvm-project

e6ebf8f9 - [mlir] Migrate away from ArrayRef(std::nullopt) (NFC) (#145140)

[mlir] Migrate away from ArrayRef(std::nullopt) (NFC) (#145140) 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 takes care of the mlir side of the migration, starting with straightforward places like "return std::nullopt;" and ternally expressions involving std::nullopt.