[swift-evolution] [Draft] Abolish IUO type

[Brent Royal-Gordon]

> It seems like this unnecessarily complicates the surface model of Swift.  To me, it seems like there is no advantage to having two ways to spell this.

	@autounwrapped let foo = functionReturningOptional()

I *believe* that, without the `@autounwrapped` attribute, there's no way to go from T? to T! without actually restating T somewhere.

-- 
Brent Royal-Gordon
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