Merge pull request #2413 from nxtstep/fix/mutex-free

[Swift-runtime] Make sure mutexes are being freed accordingly

runtime/Swift/Sources/Antlr4/misc/utils/Mutex.swift

@@ -35,4 +35,8 @@ class Mutex {
         return try closure()
     }
 
+    deinit {
+        // free the mutex resource
+        pthread_mutex_destroy(&mutex)
+    }
 }