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

When using the vim syntax for mir, an error occurs in nvim (unknown if it
affects the original vim) when opening multiple .mir buffers. Add '!' when
defining HiLink to redefine the command instead of giving an error. Only the
changes to mir.vim and llvm.vim are required to fix the error, but from
similarity making the change in all the syntax files seems prudent.

Author: Sisyph

llvm/utils/vim/syntax/llvm.vim

@@ -246,9 +246,9 @@ syn match  llvmSpecialComment /;\s*XFAIL:.*$/
 if version >= 508 || !exists("did_c_syn_inits")
   if version < 508
     let did_c_syn_inits = 1
-    command -nargs=+ HiLink hi link <args>
+    command! -nargs=+ HiLink hi link <args>
   else
-    command -nargs=+ HiLink hi def link <args>
+    command! -nargs=+ HiLink hi def link <args>
   endif
 
   HiLink llvmType Type

llvm/utils/vim/syntax/machine-ir.vim

@@ -23,9 +23,9 @@ syn match   mirPReg /$[-a-zA-Z$._][-a-zA-Z$._0-9]*/
 if version >= 508 || !exists("did_c_syn_inits")
   if version < 508
     let did_c_syn_inits = 1
-    command -nargs=+ HiLink hi link <args>
+    command! -nargs=+ HiLink hi link <args>
   else
-    command -nargs=+ HiLink hi def link <args>
+    command! -nargs=+ HiLink hi def link <args>
   endif
 
   HiLink mirType Type

llvm/utils/vim/syntax/mir.vim

@@ -37,9 +37,9 @@ syn match  mirSpecialComment /#\s*XFAIL:.*$/
 if version >= 508 || !exists("did_c_syn_inits")
   if version < 508
     let did_c_syn_inits = 1
-    command -nargs=+ HiLink hi link <args>
+    command! -nargs=+ HiLink hi link <args>
   else
-    command -nargs=+ HiLink hi def link <args>
+    command! -nargs=+ HiLink hi def link <args>
   endif
 
   HiLink mirSpecialComment SpecialComment

llvm/utils/vim/syntax/tablegen.vim

@@ -33,9 +33,9 @@ syn region  tgComment2 matchgroup=tgComment2  start=+/\*+ end=+\*/+ contains=tgT
 if version >= 508 || !exists("did_c_syn_inits")
   if version < 508
     let did_c_syn_inits = 1
-    command -nargs=+ HiLink hi link <args>
+    command! -nargs=+ HiLink hi link <args>
   else
-    command -nargs=+ HiLink hi def link <args>
+    command! -nargs=+ HiLink hi def link <args>
   endif
 
   HiLink tgKeyword Statement