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

[Guillaume Papin]

  The overview will appear on top in the "OVERVIEW:" section while the extra help is displayed at the bottom (usually after the big CommonOptionsParser::HelpMessage).

  I think both are useful, for example the overview is nicely used in clang-format (short explanation of the tool) and the extra-help has its usefulness as well in cpp11-migrate, we use it to print some simple examples and the common tooling help message.

  I guess some tools might want both possibilities.

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