[swift-evolution] Add a while clause to for loops

[Brent Royal-Gordon]

> I haven’t really run into the need for this yet personally (though I use for-in-where fairly often).

Yeah.

I haven't weighed in on the `while` thing yet, but, other than its potential usefulness in emulating C-style `for`, I'm just not convinced it's a thing people need that often. Has anyone tried to see how common it is to have a `for` loop whose first statement is an `if` or `guard` and whose body is simply a `break`? That'd be a little complex to search for, but I think it would be pretty helpful in evaluating the need, or lack thereof, for something like `while`.

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Brent Royal-Gordon
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