[vim] Set commentstring for TableGen files (#182654)

Neovim supports [commenting and uncommenting
lines](https://neovim.io/doc/user/various.html#commenting) based on what
the 'commentstring' option is set to, but this isn't set for TableGen
files. The filetype plugin for C++ built into both vim and neovim sets
'commentstring' to `// %s`, so use that in the TableGen filetype plugin
as well.

llvm/utils/vim/ftplugin/tablegen.vim

@@ -10,3 +10,4 @@ let b:did_ftplugin = 1
 setlocal matchpairs+=<:>
 setlocal softtabstop=2 shiftwidth=2
 setlocal expandtab
+setlocal commentstring=//\ %s