[vim] set commentstring (#71838)

I recently moved from using vim to lunarvim. I noticed that `gcc` (lol)
wasn't able to comment out lines, because commentstring is not set.

Thanks to Chase Colman for the suggestion in
LunarVim/LunarVim#4394.

llvm/utils/vim/ftplugin/llvm.vim

@@ -10,3 +10,4 @@ let b:did_ftplugin = 1
 setlocal softtabstop=2 shiftwidth=2
 setlocal expandtab
 setlocal comments+=:;
+setlocal commentstring=;\ %s