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

[James Henderson via llvm-commits]

jh7370 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).

The "ForWrite" test is still failing on Windows.

Could we not just fix both the tests for Windows rather than disabling it? I'm pretty sure the behaviour should be the same, just possibly a different error code. 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.

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