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

[A. Jiang via llvm-commits]

================
@@ -0,0 +1,18 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <ciso646>
+
+// check that <ciso646> is removed in C++20
+// When built with modules, <ciso646> should be omitted.
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: clang-modules-build
----------------
frederick-vs-ja wrote:

I'm afraid that currently we can't reliably detect `-fmodules`, as `__has_feature(modules)` is currently `1` in C++20 mode even when `-fno-modules` is used, see #57432.

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