[stdlib] Change C-style for loop to Swift-style for-in loop

stdlib/public/core/CString.swift

@@ -60,7 +60,7 @@ public func _persistCString(s: UnsafePointer<CChar>) -> [CChar]? {
   }
   let length = Int(_swift_stdlib_strlen(s))
   var result = [CChar](count: length + 1, repeatedValue: 0)
-  for var i = 0; i < length; i += 1 {
+  for i in 0..<length {
     // FIXME: this will not compile on platforms where 'CChar' is unsigned.
     result[i] = s[i]
   }