DO REPEAT: Properly convert lex_syntax_mode to segmenter_mode.

Clang reported that "enum lex_syntax_mode" was being implicitly converted
to "enum segmenter_mode".  Luckily, equal values in each enum have the
same meaning, but it seems better to convert by hand, so this patch
implements that.

Reported-by: Jeremy Lavergne <jeremy@lavergne.gotdns.org>