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

[Tim Vermeulen via swift-evolution]

We can already use a where clause in a for loop like this:

for element in array where someCondition(element) {
    // …
}

which basically acts like

for element in array {
    guard someCondition(element) else { continue }
    // …
}

Sometimes you want to break out of the loop when the condition isn’t met 
instead. I propose a while clause:

for element in array while someCondition(element) {
    // …
}

which would be syntactic sugar for

for element in array {
    guard someCondition(element) else { break }
    …
}

I can see this particularly being useful if we have a sorted array and we 
already know that once the condition isn’t met, it won’t be met either for 
subsequent elements. Another use case could be an infinite sequence that we 
want to cut off somewhere (which is simply not possible using a where clause).
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