[libc++] Optimize ranges::fill{,_n} for vector<bool>::iterator (#84642)

```
------------------------------------------------------
Benchmark                          old             new
------------------------------------------------------
bm_ranges_fill_n/1             1.64 ns         3.06 ns
bm_ranges_fill_n/2             3.45 ns         3.06 ns
bm_ranges_fill_n/3             4.88 ns         3.06 ns
bm_ranges_fill_n/4             6.46 ns         3.06 ns
bm_ranges_fill_n/5             8.03 ns         3.06 ns
bm_ranges_fill_n/6             9.65 ns         3.07 ns
bm_ranges_fill_n/7             11.5 ns         3.06 ns
bm_ranges_fill_n/8             13.0 ns         3.06 ns
bm_ranges_fill_n/16            25.9 ns         3.06 ns
bm_ranges_fill_n/64             103 ns         4.62 ns
bm_ranges_fill_n/512            711 ns         4.40 ns
bm_ranges_fill_n/4096          5642 ns         9.86 ns
bm_ranges_fill_n/32768        45135 ns         33.6 ns
bm_ranges_fill_n/262144      360818 ns          243 ns
bm_ranges_fill_n/1048576    1442828 ns          982 ns
bm_ranges_fill/1               1.63 ns         3.17 ns
bm_ranges_fill/2               3.43 ns         3.28 ns
bm_ranges_fill/3               4.97 ns         3.31 ns
bm_ranges_fill/4               6.53 ns         3.27 ns
bm_ranges_fill/5               8.12 ns         3.33 ns
bm_ranges_fill/6               9.76 ns         3.32 ns
bm_ranges_fill/7               11.6 ns         3.29 ns
bm_ranges_fill/8               13.2 ns         3.26 ns
bm_ranges_fill/16              26.3 ns         3.26 ns
bm_ranges_fill/64               104 ns         4.92 ns
bm_ranges_fill/512              716 ns         4.47 ns
bm_ranges_fill/4096            5772 ns         8.21 ns
bm_ranges_fill/32768          45778 ns         33.1 ns
bm_ranges_fill/262144        351422 ns          241 ns
bm_ranges_fill/1048576      1404710 ns          965 ns
```

2 files changed, 50 insertions, 0 deletions

diff --git a/libcxx/benchmarks/CMakeLists.txt b/libcxx/benchmarks/CMakeLists.txt
index b436e96..3dec6fa 100644
--- a/libcxx/benchmarks/CMakeLists.txt
+++ b/libcxx/benchmarks/CMakeLists.txt
@@ -176,6 +176,7 @@ set(BENCHMARK_TESTS
     algorithms/count.bench.cpp
     algorithms/equal.bench.cpp
     algorithms/find.bench.cpp
+    algorithms/fill.bench.cpp
     algorithms/for_each.bench.cpp
     algorithms/lower_bound.bench.cpp
     algorithms/make_heap.bench.cpp
diff --git a/libcxx/benchmarks/algorithms/fill.bench.cpp b/libcxx/benchmarks/algorithms/fill.bench.cpp
new file mode 100644
index 0000000..40f3742
--- /dev/null
+++ b/libcxx/benchmarks/algorithms/fill.bench.cpp
@@ -0,0 +1,49 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <algorithm>
+#include <benchmark/benchmark.h>
+#include <vector>
+
+static void bm_fill_n(benchmark::State& state) {
+  std::vector<bool> vec1(state.range());
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(vec1);
+    benchmark::DoNotOptimize(std::fill_n(vec1.begin(), vec1.size(), false));
+  }
+}
+BENCHMARK(bm_fill_n)->DenseRange(1, 8)->Range(16, 1 << 20);
+
+static void bm_ranges_fill_n(benchmark::State& state) {
+  std::vector<bool> vec1(state.range());
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(vec1);
+    benchmark::DoNotOptimize(std::ranges::fill_n(vec1.begin(), vec1.size(), false));
+  }
+}
+BENCHMARK(bm_ranges_fill_n)->DenseRange(1, 8)->Range(16, 1 << 20);
+
+static void bm_fill(benchmark::State& state) {
+  std::vector<bool> vec1(state.range());
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(vec1);
+    std::fill(vec1.begin(), vec1.end(), false);
+  }
+}
+BENCHMARK(bm_fill)->DenseRange(1, 8)->Range(16, 1 << 20);
+
+static void bm_ranges_fill(benchmark::State& state) {
+  std::vector<bool> vec1(state.range());
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(vec1);
+    benchmark::DoNotOptimize(std::ranges::fill(vec1, false));
+  }
+}
+BENCHMARK(bm_ranges_fill)->DenseRange(1, 8)->Range(16, 1 << 20);
+
+BENCHMARK_MAIN();