[llvm] [Support] Report EISDIR when opening a directory (PR #79880)

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

> If you need me to look into it because you don't have access to a Windows machine, I might be able to, assuming I can get current LLVM to build.

That would help a lot. We don't have Windows build environments handy.

https://github.com/llvm/llvm-project/pull/79880