[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
#  if defined(_LIBCPP_FILESYSTEM_HAS_FSTREAM_FALLBACK)
```

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