Improve cstyle performance

With this patch I can notice 7x-8x improvement in performance. The main
culprit was `tryMultiLineStringLiteral` which searched the whole document
for a multiline literal beginning. I have now limited it to 25 lines max.

Besides, the multiline string matching is useless for every other language
besides C++ so return early if the mode is no C++

CCBUG: 465841
CCBUG: 466531