[NFC] Always lock free test: add indirection

I have a big patch coming up, and this indirection is required to avoid hitting the following after my big change:

  error: empty struct has size 0 in C, size 1 in C++ [-Werror,-Wextern-c-compat]

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@350772 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed