libclang based doxygen replacement

Clang Frontend
1

A new github repo for the clang_doc project can be found here:

https://github.com/donhinton/clang-tools-extra

I also added a gen-docs target to llvm/Makefile.rules, so you
can run it. It can be found here:

https://github.com/donhinton/llvm

thanks...
don