[PATCH] Warn on suspicious increments/decrements in for loops

[Richard Trieu]

  @jordan_rose, I want this warning.  Not sure about other people

  @gribozavr, earlier versions of this did trigger on LLVM and Clang.  The warning has been fine-tuned since then to avoid those false positives.

  Also, I seemed to have messed up the indentation when I wrote the visitors for the first -Wloop-analysis warning and managed to copy the bad indentation over to this change.  I will go fix them.

http://llvm-reviews.chandlerc.com/D628