[libc++] Fix benchmark failure

NOKEYCHECK=True
GitOrigin-RevId: a2afc8249a9950da861bce5fd724c1ccf6c4eea4
diff --git a/benchmarks/filesystem.bench.cpp b/benchmarks/filesystem.bench.cpp
index 95aecd5..4fe4fc5 100644
--- a/benchmarks/filesystem.bench.cpp
+++ b/benchmarks/filesystem.bench.cpp
@@ -83,7 +83,7 @@
     PP /= Part;
   benchmark::DoNotOptimize(PP.native().data());
   while (st.KeepRunning()) {
-    for (auto &E : PP) {
+    for (auto const& E : PP) {
       benchmark::DoNotOptimize(E.native().data());
     }
     benchmark::ClobberMemory();
@@ -104,7 +104,7 @@
   benchmark::DoNotOptimize(PP.native().data());
   while (st.KeepRunning()) {
     const path P = PP.native();
-    for (auto &E : P) {
+    for (auto const& E : P) {
       benchmark::DoNotOptimize(E.native().data());
     }
     benchmark::ClobberMemory();