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

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

@azhan92, the failure on Windows looks real too:
```
LLVM-Unit :: Support/./SupportTests.exe/FileSystemTest/OpenDirectoryAsFileForRead
```

I think we may just want to find a way to not run the test on Windows (e.g., use a macro guard to leave it out of the Windows build).

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