[libc++] Implement P0339R6 (polymorphic_allocator<> as a vocabulary type)

Reviewed By: ldionne, #libc

Spies: LRFLEW, libcxx-commits, arichardson, krytarowski, jdoerfert

Differential Revision: https://reviews.llvm.org/D137739

NOKEYCHECK=True
GitOrigin-RevId: 181cce6b696e032735fb3e3d23b1db1fb9fca5cb
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index f37751b..a04eacb 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -48,6 +48,7 @@
 - P0482R6 - char8_t: A type for UTF-8 characters and strings
 - P2438R2 - ``std::string::substr() &&``
 - P0600R1 - ``nodiscard`` in the library
+- P0339R6 - ``polymorphic_allocator<>`` as a vocabulary type
 
 Improvements and New Features
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