[PATCH] Allow Clang tools to display an overview when using the CommonOptionsParser.

[Guillaume Papin]

  No, not really. Of course cpp11-migrate will probably end-up using it sooner or later but I submitted this patch because I really think some tools might want it and I don't see how it can hurt tools who don't (I realized this was missing when adding some help to cpp11-migrate, I guess I should have submitted at that time...).

  Looking at the binaries produced in build/bin it doesn't seem uncommon to put an overview, I don't see why Clang tools will be different.

http://llvm-reviews.chandlerc.com/D1185