[pstl] Gate usage of TBB in test on whether TBB is used as a backend

__PSTL_USE_PAR_POLICIES is the wrong macro to use there, since the PSTL
could conceivably be using parallel policies but not TBB as a backend.

llvm-svn: 356935

pstl/test/test_partial_sort.cpp

@@ -79,7 +79,7 @@ struct test_brick_partial_sort
             if (m1 - first > 1)
             {
                 auto complex = std::ceil(n * std::log(float32_t(m1 - first)));
-#if __PSTL_USE_PAR_POLICIES
+#if defined(__PSTL_PAR_BACKEND_TBB)
                 auto p = tbb::this_task_arena::max_concurrency();
 #else
                 auto p = 1;