Add complete_check() to ccomplete completion script to avoid UI hangs
and keep Vim responsive as ccomplete can be slow on huge files.
closes: vim/vim#178263add0d5e75
vim-patch:44309b9: runtime(ccomplete): return partial results on complete_check()
closes: vim/vim#1783844309b9d08
Co-authored-by: Maxim Kim <habamax@gmail.com>