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

[Ben Gertzfield]

  OK, I added the tests. Not sure I quite know what I'm doing, but it's a start:

  http://llvm-reviews.chandlerc.com/differential/diff/866/

  The command-line argument forwarding one passes, I think, but the XML one doesn't, and I'm not sure why. Test result:

  http://llvm-reviews.chandlerc.com/P45

  Output from tools/clang/test/Index/Output/comment-custom-block-command.cpp.tmp/out.c-index-direct :

  http://llvm-reviews.chandlerc.com/P46

  Output from tools/clang/test/Index/Output/comment-custom-block-command.cpp.tmp/out.c-index-pch :

  https://phabricator.fb.com/P1951541

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