Host auto-updated Javadoc (Java) and doxygen (C/C++) pages online

_From @gharrma on May 25, 2018 18:58_

I don't have any experience with this, but hosting a Javadoc/doxygen page would allow us to link directly to source classes within the README files and on the website.

_Copied from original issue: gharrma/polyllvm#58_