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

[Louis Dionne 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
----------------
ldionne wrote:

IIUC, these diagnostics are not emitted under Clang modules? So then the clever using _LIBCPP_DEPRECATED(...)` approach is equivalent to

```
#if !__has_feature(cxx_modules) // whatever the right way of checking that is
# warning "this header is deprecated"
#endif
```

I'm not super strongly attached, but I do think that using a plain `#warning` makes this easier to understand than the clever `using` trick.

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