parse doxygen comments for macros with libclang.

I'm not entirely happy about this script, but as it saves
a lot of work in keeping the docs up to date with the
actual code I think checking it in makes sense.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162690 91177308-0d34-0410-b5e6-96231b3b80d8