Summary:
- move unit/clang/ to clang/unit/ and make it a dune library
- move unit/nullsafe/ to nullsafe/unit/ and make it a dune library
- make unit/ a dune library
- inline most of dune.common.in into dune.in and make more explicit
rules for each binary as they don't depend on the same libraries
- move inferunit from unit/ to ./ like the other toplevel binaries
Reviewed By: skcho
Differential Revision: D21440822
fbshipit-source-id: 075c693e0