[libcxx-commits] [libcxx] [libc++] Don't use `copy_file_impl_fstream` when not declared (PR #214706)

[Louis Dionne via libcxx-commits]

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ldionne wrote:

```suggestion
// sendfile and copy_file_range need to fall back
// to the fstream implementation for special files
#if _LIBCPP_HAS_LOCALIZATION
#  include <fstream>
#  define _LIBCPP_FILESYSTEM_HAS_FSTREAM_FALLBACK
#endif
```

https://github.com/llvm/llvm-project/pull/214706