Removes redundant buffer zeroing in _persistCString func

stdlib/public/Darwin/Foundation/NSStringAPI.swift

@@ -73,10 +73,10 @@ internal func _persistCString(_ p: UnsafePointer<CChar>?) -> [CChar]? {
   guard let cString = p else {
     return nil
   }
-  let len = UTF8._nullCodeUnitOffset(in: cString)
-  var result = [CChar](repeating: 0, count: len + 1)
-  for i in 0..<len {
-    result[i] = cString[i]
+  let bytesToCopy = UTF8._nullCodeUnitOffset(in: cString) + 1 // +1 for the terminating NUL
+  let result = [CChar](unsafeUninitializedCapacity: bytesToCopy) { buffer, initializedCount in
+      buffer.baseAddress!.assign(from: cString, count: bytesToCopy)
+      initializedCount = bytesToCopy
   }
   return result
 }