Swift 5 version: Use UnsafeRawBufferPointer as type of the closure argument, and bytes.baseAddress to pass address to the Common Crypto function:

import Foundation
import CommonCrypto

extension String {
    var md5: String {
        let data = Data(self.utf8)
        let hash = data.withUnsafeBytes { (bytes: UnsafeRawBufferPointer) -> [UInt8] in
            var hash = [UInt8](repeating: 0, count: Int(CC_MD5_DIGEST_LENGTH))
            CC_MD5(bytes.baseAddress, CC_LONG(data.count), &hash)
            return hash
        }
        return hash.map { String(format: "%02x", $0) }.joined()
    }
}

(Note that the conversion of a string to UTF-8 data cannot fail, there is no need to return an optional.)

CC_MD5 has been deprecated with the iOS 13. Instead, you can use CC_SHA256.

In iOS 13 and above there is a framework CryptoKit which is a wrapper around CommonCrypto framework and around the MD5 hash function.

import CryptoKit

let d = "Hello"
let r = Insecure.MD5.hash(data: d.data(using: .utf8)!)
print(r)

/*Output: MD5 digest: 8b1a9953c4611296a827abf8c47804d7*/

In iOS 13 and above there is a framework CryptoKit. Try using this:

extension Data {       
    var md5: String {
        Insecure.MD5
            .hash(data: self)
            .map {String(format: "%02x", $0)}
            .joined()
    }
}