[libcxx-commits] [libcxx] [libc++] Fix header deprecations (PR #163356)

[Nico Weber via libcxx-commits]

nico wrote:

This emits warnings for deprecated headers pulled in by system headers, e.g.:

```
In file included from ../../ios/chrome/browser/shared/ui/util/uikit_ui_util.mm:7:
In file included from sdk/xcode_links/iPhoneSimulator26.0.sdk/System/Library/Frameworks/Accelerate.framework/Headers/Accelerate.h:20:
In file included from sdk/xcode_links/iPhoneSimulator26.0.sdk/System/Library/Frameworks/Accelerate.framework/Headers/../Frameworks/vecLib.framework/Headers/vecLib.h:111:
In file included from sdk/xcode_links/iPhoneSimulator26.0.sdk/System/Library/Frameworks/Accelerate.framework/Frameworks/vecLib.framework/Headers/vForce.h:50:
gen/third_party/libc++/src/include/ciso646:28:6: error: <ciso646> is removed in C++20. Include <version> instead. [-Werror,-W#warnings]
   28 | #    warning <ciso646> is removed in C++20. Include <version> instead.
```

That's not actionable for users.

What's a good path forward here?

https://github.com/llvm/llvm-project/pull/163356