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

[Fangrui Song via llvm-commits]

MaskRay wrote:

On Linux, `MemoryBuffer::getFile` on a directory returns an error `EISDIR`. `fstat` is called `getOpenFileImpl<MB>` after a file descriptor is obtained.

I am concerned that this change would cause two `fstat` syscalls for operations like `MemoryBuffer::getFile`. This perhaps needs `#ifdef` like MVS below.

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