[vim] Fix command already exists on opening multiple mir buffers

When using the vim syntax for mir, an error occurs in nvim  when opening
multiple .mir buffers. delcommand HiLink in the mir syntax file to avoid
the issue.

Author: Sisyph

llvm/utils/vim/syntax/mir.vim

@@ -43,6 +43,8 @@ if version >= 508 || !exists("did_c_syn_inits")
   endif
 
   HiLink mirSpecialComment SpecialComment
+
+  delcommand HiLink
 endif
 
 let b:current_syntax = "mir"