[libc++abi] Refactor around __dynamic_cast

This commit contains refactorings around __dynamic_cast without changing
its behavior. Some important changes include:

- Refactor __dynamic_cast into various small helper functions;
- Move dynamic_cast_stress.pass.cpp to libcxx/benchmarks and refactor
  it into a benchmark. The benchmark performance numbers are updated
  as well.

Differential Revision: https://reviews.llvm.org/D138006

Author: Lancern

libcxx/benchmarks/CMakeLists.txt

@@ -183,6 +183,7 @@ set(BENCHMARK_TESTS
     algorithms/sort_heap.bench.cpp
     algorithms/stable_sort.bench.cpp
     libcxxabi/dynamic_cast.bench.cpp
+    libcxxabi/dynamic_cast_old_stress.bench.cpp
     allocation.bench.cpp
     deque.bench.cpp
     deque_iterator.bench.cpp

libcxx/benchmarks/libcxxabi/dynamic_cast_old_stress.bench.cpp

@@ -0,0 +1,77 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 <cassert>
+#include <cstddef>
+#include <utility>
+
+#include "benchmark/benchmark.h"
+
+template <std::size_t Indx, std::size_t Depth>
+struct C : public virtual C<Indx, Depth - 1>, public virtual C<Indx + 1, Depth - 1> {
+  virtual ~C() {}
+};
+
+template <std::size_t Indx>
+struct C<Indx, 0> {
+  virtual ~C() {}
+};
+
+template <std::size_t Indx, std::size_t Depth>
+struct B : public virtual C<Indx, Depth - 1>, public virtual C<Indx + 1, Depth - 1> {};
+
+template <class Indx, std::size_t Depth>
+struct makeB;
+
+template <std::size_t... Indx, std::size_t Depth>
+struct makeB<std::index_sequence<Indx...>, Depth> : public B<Indx, Depth>... {};
+
+template <std::size_t Width, std::size_t Depth>
+struct A : public makeB<std::make_index_sequence<Width>, Depth> {};
+
+constexpr std::size_t Width = 10;
+constexpr std::size_t Depth = 5;
+
+template <typename Destination>
+void CastTo(benchmark::State& state) {
+  A<Width, Depth> a;
+  auto base = static_cast<C<Width / 2, 0>*>(&a);
+
+  Destination* b = nullptr;
+  for (auto _ : state) {
+    b = dynamic_cast<Destination*>(base);
+    benchmark::DoNotOptimize(b);
+  }
+
+  assert(b != 0);
+}
+
+BENCHMARK(CastTo<B<Width / 2, Depth>>);
+BENCHMARK(CastTo<A<Width, Depth>>);
+
+BENCHMARK_MAIN();
+
+/**
+ * Benchmark results: (release builds)
+ *
+ * libcxxabi:
+ * ----------------------------------------------------------------------
+ * Benchmark                            Time             CPU   Iterations
+ * ----------------------------------------------------------------------
+ * CastTo<B<Width / 2, Depth>>       1997 ns         1997 ns       349247
+ * CastTo<A<Width, Depth>>            256 ns          256 ns      2733871
+ *
+ * libsupc++:
+ * ----------------------------------------------------------------------
+ * Benchmark                            Time             CPU   Iterations
+ * ----------------------------------------------------------------------
+ * CastTo<B<Width / 2, Depth>>       5240 ns         5240 ns       133091
+ * CastTo<A<Width, Depth>>            866 ns          866 ns       808600
+ *
+ *
+ */