Upgrade to 2022-era LLVM LIBCXX

third_party/libcxx/experimental/unordered_set

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+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_EXPERIMENTAL_UNORDERED_SET
+#define _LIBCPP_EXPERIMENTAL_UNORDERED_SET
+
+/*
+    experimental/unordered_set synopsis
+
+// C++1z
+namespace std {
+namespace experimental {
+inline namespace fundamentals_v1 {
+namespace pmr {
+
+  template <class T, class Hash = hash<T>, class Pred = equal_to<T>>
+  using unordered_set = std::unordered_set<T, Hash, Pred,
+                       polymorphic_allocator<T>>;
+
+  template <class T, class Hash = hash<T>, class Pred = equal_to<T>>
+  using unordered_multiset = std::unordered_multiset<T, Hash, Pred,
+                       polymorphic_allocator<T>>;
+
+} // namespace pmr
+} // namespace fundamentals_v1
+} // namespace experimental
+} // namespace std
+
+ */
+
+#include <__assert> // all public C++ headers provide the assertion handler
+#include <experimental/__config>
+#include <experimental/memory_resource>
+#include <unordered_set>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_LFTS_PMR
+
+#ifndef _LIBCPP_CXX03_LANG
+
+template <class _Value,
+          class _Hash = hash<_Value>, class _Pred = equal_to<_Value>>
+using unordered_set = _VSTD::unordered_set<_Value, _Hash, _Pred,
+                        polymorphic_allocator<_Value>>;
+
+template <class _Value,
+          class _Hash = hash<_Value>, class _Pred = equal_to<_Value>>
+using unordered_multiset = _VSTD::unordered_multiset<_Value, _Hash, _Pred,
+                        polymorphic_allocator<_Value>>;
+
+#endif // _LIBCPP_CXX03_LANG
+
+_LIBCPP_END_NAMESPACE_LFTS_PMR
+
+#endif /* _LIBCPP_EXPERIMENTAL_UNORDERED_SET */