[PATCH] Add CommentOptions, allow specifying custom comment block commands

[Ben Gertzfield]

  > Oh, and one more thing. Please add two tests:

  Sure, no problem. I duplicated retain-comments-from-system-headers.c to another file in the same directory (retain-comment-block-commands.c), but 'make check' didn't seem to build/run it.

  What do I need to do to build/run the test? I'm using cmake, FYI.

http://llvm-reviews.chandlerc.com/D272