[stdlib] Backward compatibility fix for a flatMap on [String]

[moiseev]

Since String started to conform to Collection, the flatMap with a
sequence returning closure is now a better match than the one that
relies on the optional promotion in this code:

[""].flatMap { $0 }

which results in the default type of this expression changing from
[String] to [Character].

Restoring the old behavior in Swift 3 mode by adding a very explicit
overload.

Fixes: rdar://problem/32024978

[moiseev]

/cc @dabrahams , @airspeedswift

[moiseev]

@swift-ci Please test

[moiseev]

@swift-ci Please Test Source Compatibility

[moiseev]

@swift-ci Please smoke test

[moiseev]

@swift-ci Please Test Source Compatibility

[moiseev]

@swift-ci Please Test Source Compatibility

[moiseev]

@swift-ci Please smoke test

[Coeur]

Fixes: rdar://problem/32024978

Can someone fill http://www.openradar.me/32024978 or provide a public resource?

[moiseev]

@Coeur The description of this PR explains it, doesn't it?

The following code:

struct S { var p: String }

func f(xs: [S]) -> [String] {
   return xs.flatMap { $0.p }
}

would not compile once String becomes Collection without this fix, with the error being something like 'cannot convert [Character] to return type [String]'.