[swift-evolution] [Draft] Abolish IUO type

[Brent Royal-Gordon]

> (Come to think of it, is there a cost to converting between `T?` and `T!`, or do they get optimized into the same thing?)

I can't imagine there would be. @autounwrapped (or whatever) is a purely compile-time thing which tells the compiler to automatically insert a `!` operator if it will make the expression typecheck. Other than that compile-time behavior, `!` is an exact synonym for `?`.

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Brent Royal-Gordon
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