avoid aggressive optimization

Summary: GCC will now dead-code eliminate the folly-based version of this test without a mechanisms to force the result to be live.

Reviewed By: ttsugriy

Differential Revision: D2854633

fb-gh-sync-id: 0e3841ed22c040fda7653bcfb5a3f19ca3d1f835

folly/test/AsciiCaseInsensitiveBenchmark.cpp

@@ -100,6 +100,7 @@ inline void test_operator_on_search(int iters) {
       cmp
     ) - lorem_ipsum.begin();
   }
+  doNotOptimizeAway(dummy);
 }
 
 BENCHMARK(LegacyCaseInsensitiveCheck, iters) {