[libc++] Fix minor warnings in libcxx benchmarks

This fixes some unused variable and "comparison of integers of different
signs" warnings in the benchmarks.

Differential Revision: https://reviews.llvm.org/D156613

Author: ilvokhin

libcxx/benchmarks/allocation.bench.cpp

@@ -76,8 +76,8 @@ static void BM_DeallocateOnly(benchmark::State& st) {
     p = AllocWrapper::Allocate(alloc_size);
   }
 
-  void** Data      = Pointers.data();
-  void** const End = Pointers.data() + Pointers.size();
+  void** Data                       = Pointers.data();
+  [[maybe_unused]] void** const End = Pointers.data() + Pointers.size();
   while (st.KeepRunning()) {
     AllocWrapper::Deallocate(*Data, alloc_size);
     Data += 1;

libcxx/benchmarks/monotonic_buffer.bench.cpp

@@ -16,7 +16,7 @@ static void bm_list(benchmark::State& state) {
   std::pmr::monotonic_buffer_resource resource(buffer, sizeof(buffer));
   for (auto _ : state) {
     std::pmr::list<int> l(&resource);
-    for (size_t i = 0; i != state.range(); ++i) {
+    for (int64_t i = 0; i != state.range(); ++i) {
       l.push_back(1);
       benchmark::DoNotOptimize(l);
     }