[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]

llvm-svn: 350772

libcxx/test/std/atomics/atomics.lockfree/isalwayslockfree.pass.cpp

@@ -59,7 +59,7 @@ void checkLongLongTypes() {
   static_assert((0 != ATOMIC_LLONG_LOCK_FREE) == ExpectLockFree, "");
 }
 
-int main()
+void run()
 {
 // structs and unions can't be defined in the template invocation.
 // Work around this with a typedef.
@@ -134,3 +134,5 @@ int main()
     static_assert(std::atomic<void*>::is_always_lock_free == (2 == ATOMIC_POINTER_LOCK_FREE));
     static_assert(std::atomic<std::nullptr_t>::is_always_lock_free == (2 == ATOMIC_POINTER_LOCK_FREE));
 }
+
+int main() { run(); }