clang-format: Increase penalty for breaking comments.

Unexpectedly, it seems that people commonly know what they were doing
when writing a comment.

Also, being more conservative about comment breaking has the advantage
of giving more flexibility. If a linebreak within the comment can
improve formatting, the author can add it (after which clang-format
won't undo it). There is no way to override clang-format's behavior if
it breaks a comment.

llvm-svn: 197698