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

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

While I can understand the motivation behind this change (it can't possibly make sense to open a directory for reading, right?) I'd be concerned about collateral damage resulting from this patch, where somebody might be expecting to be able to open directories using this interface on specific systems.

There's also a minor time of check to time of use issue with this patch, since it could be possible to replace a file with a directory being the new check and the read, although I'm less concerned about that.

Thoughts?

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