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

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

@azhan92, this test failure in the Linux CI results is probably real and looks related to the change:
```
Failed Tests (1):
  LLVM-Unit :: Support/./SupportTests/FileSystemTest/OpenDirectoryAsFile
```

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