[libcxx] [llvm] [libc++] Deprecate and remove meaningless `<cxxx>` headers (PR #111615)

[A. Jiang via llvm-commits]

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@@ -21,4 +21,9 @@
 #  pragma GCC system_header
 #endif
 
+#if _LIBCPP_STD_VER >= 20
+using __standard_header_ciso646 _LIBCPP_DEPRECATED_("removed in C++20") = void;
+using __use_standard_header_ciso646                                     = __standard_header_ciso646;
+#endif
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frederick-vs-ja wrote:

`_LIBCPP_ENABLE_REMOVED_CISO646` added.

https://github.com/llvm/llvm-project/pull/111615