Revert "Allow relockable scopes with thread safety attributes."

This reverts commit r339456.

The change introduces a new crash, see

class SCOPED_LOCKABLE FileLock {
 public:
  explicit FileLock()
      EXCLUSIVE_LOCK_FUNCTION(file_);
  ~FileLock() UNLOCK_FUNCTION(file_);
  void Lock() EXCLUSIVE_LOCK_FUNCTION(file_);
  Mutex file_;
};

void relockShared2() {
  FileLock file_lock;
  file_lock.Lock();
}

llvm-svn: 339558