[split-file] Migrate to OptTable (#224966)
split-file is one of three tools that parse with
cl::ParseCommandLineOptions(LongOptionsUseDoubleDash=true). Parse its
options with an OptTable instead, so that the mode can be removed from
cl::. --no-leading-lines now overrides an earlier --leading-lines
rather than being ignored, and `--help` lists only split-file's
options. `--version` still exits 0, which libc++'s test configuration
relies on to detect the tool.
LLM-aided
