[PATCH] Added doxygen support for the tools of clang-tools-extra repo

[Sean Silva]

  Ugh. This is just repeating the same mistakes that are currently present in the llvm and clang doxygen setups.

  In particular, the Makefile is unusable for CMake builds (and AFAIK nobody uses the functionality in that Makefile anyway). It also uses LLVM's weird doxygen stylesheet instead of the default one which is *much* easier to navigate.

  We should really just have a single Doxyfile (maybe Doxyfile.in, but make it work with CMake and Make). I see no reason to add anything else. You can look at <https://github.com/chisophugis/klang/tree/master/docs> for an example of my current preference for the layout of the `docs/` directory.

  I also noticed that the Sphinx Makefile is called Makefile.sphinx. I've added some post-commit review to the commit that added that (I should have noticed this earlier during pre-commit review).

  Let's avoid making the mistakes of the past.

http://llvm-reviews.chandlerc.com/D451