[llvm] Avoid calling `GetFileAttributesW` in Windows' `fs::access` when checking for existence (PR #83495)

[Alexandre Ganea via llvm-commits]

aganea wrote:

Regardless I think it would still be valuable if we returned `errc::no_such_file_or_directory` on failure, when calling `access` with `AccessMode::Exists`, and not some other Windows error code, ie:
```
if (Attributes == INVALID_FILE_ATTRIBUTES) {
  if (Mode == AccessMode::Exists)
    return errc::no_such_file_or_directory;
...
```

https://github.com/llvm/llvm-project/pull/83495