[libc++] Add a missing include to `swap_allocator.h`.

[var-const]

Summary

Also add tests for the file.

[var-const]

Fix the CI.

libcxx/test/libcxx/memory/swap_allocator.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+
+// <memory>
+
+// template <typename _Alloc>
+// void __swap_allocator(_Alloc& __a1, _Alloc& __a2);
+
+#include <cassert>
+#include <memory>
+// Transitively includes `swap_allocator.h` (directly including it breaks the modules build).
+#include <vector>
+#include <utility>
+
+#include "test_macros.h"
+
+template <bool Propagate, bool Noexcept>
+struct Alloc {
+  int i = 0;
+  Alloc() = default;
+  Alloc(int set_i) : i(set_i) {}
+
+  using value_type = int;
+  using propagate_on_container_swap = std::integral_constant<bool, Propagate>;
+
+  friend void swap(Alloc& a1, Alloc& a2) TEST_NOEXCEPT_COND(Noexcept) {
+    std::swap(a1.i, a2.i);
+  }
+
+};
+
+using PropagatingAlloc = Alloc</*Propagate=*/true, /*Noexcept=*/true>;
+static_assert(std::allocator_traits<PropagatingAlloc>::propagate_on_container_swap::value, "");
+
+using NonPropagatingAlloc = Alloc</*Propagate=*/false, /*Noexcept=*/true>;
+static_assert(!std::allocator_traits<NonPropagatingAlloc>::propagate_on_container_swap::value, "");
+
+using NoexceptSwapAlloc = Alloc</*Propagate=*/true, /*Noexcept=*/true>;
+using ThrowingSwapAlloc = Alloc</*Propagate=*/true, /*Noexcept=*/false>;
+
+int main(int, char**) {
+  {
+    PropagatingAlloc a1(1), a2(42);
+    std::__swap_allocator(a1, a2);
+    assert(a1.i == 42);
+    assert(a2.i == 1);
+  }
+
+  {
+    NonPropagatingAlloc a1(1), a2(42);
+    std::__swap_allocator(a1, a2);
+    assert(a1.i == 1);
+    assert(a2.i == 42);
+  }
+
+#if TEST_STD_VER >= 11
+  {
+    NoexceptSwapAlloc noexcept_alloc;
+    static_assert(noexcept(std::__swap_allocator(noexcept_alloc, noexcept_alloc)), "");
+  }
+
+#if TEST_STD_VER > 11
+  { // From C++14, `__swap_allocator` is unconditionally noexcept.
+    ThrowingSwapAlloc throwing_alloc;
+    static_assert(noexcept(std::__swap_allocator(throwing_alloc, throwing_alloc)), "");
+  }
+#else
+  { // Until C++14, `__swap_allocator` is only noexcept if the underlying `swap` function is `noexcept`.
+    ThrowingSwapAlloc throwing_alloc;
+    static_assert(!noexcept(std::__swap_allocator(throwing_alloc, throwing_alloc)), "");
+  }
+#endif // TEST_STD_VER > 11
+#endif // TEST_STD_VER >= 11
+
+  return 0;
+}