[libc++] Implement P0591R4 (Utility functions to implement uses-allocator construction)

Reviewed By: ldionne, #libc, huixie90

Spies: huixie90, libcxx-commits, mgorny

Differential Revision: https://reviews.llvm.org/D131898

NOKEYCHECK=True
GitOrigin-RevId: 79df8e19beb9db218547bae5265139d0ff67343a
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 7558fd2..8a49ec4 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -41,6 +41,7 @@
 - P2417R2 - A more constexpr bitset
 - P2445R1 - ``std::forward_like``
 - P2273R3 - Making ``std::unique_ptr`` constexpr
+- P0591R4 - Utility functions to implement uses-allocator construction
 
 Improvements and New Features
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