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.
This brings the utils/vim folder into a more vim-like format by moving
the syntax hightlighting files into a syntax subdirectory. It adds
some minimal settings that everyone should agree on to ftdetect/ftplugin and
features a new indentation plugin for .ll files.
llvm-svn: 235369