[swift-evolution] Required Callback

[James Campbell]

@noescape is a hint that the callback will only be called once and allows
compiler to optimise, we may or may not be able to combine @required
functionality into that but it looks like it was rejected for now.

@required is a compiler hint / sanity check to enforce that the callback is
actually called once by the developer. There is no focus on any compiler
enhancements other than that.
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