[llvm] [vim] Fix command already exists on opening multiple mir buffers (PR #82410)

[Joe Nash via llvm-commits]

Sisyph wrote:

> I think the problem with MIR is because it doesn't have `delcommand HiLink`. If I add that command to `mir.vim`, the problem also goes away. What do you think about that fix instead?

That makes sense and sounds like a good fix. Probably better than this one since it is unlikely users would want to call HiLink. Do you want to put that change up, or should I update this one?

https://github.com/llvm/llvm-project/pull/82410