[clang] [lld] [libcxx] [llvm] [flang] [libc++][streams] P1759R6: Native handles and file streams (PR #76632)

[Mark de Wever via cfe-commits]

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+//===----------------------------------------------------------------------===//
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mordante wrote:

Not related to this file. But I miss tests for the typedefs added to the 4 classes. These are typically in the types tests. For example, libcxx/test/std/input.output/file.streams/fstreams/filebuf/types.pass.cpp. When adding the test make sure you test all the wording
```
The type native_handle_type represents a platform-specific native handle
to a file. It is trivially copyable and models semiregular.
```

https://github.com/llvm/llvm-project/pull/76632