Add a very primitive clang based multifile 'delta'.

 - Interface is more or less like multidelta.

 - Keep in mind it won't work very well; delta is O(N^2) frequently in
   practice. multidelta manages to work because it folds lines to keep
   N small, but this is just working on raw tokens. However, I have a
   fancy pants DAG based delta algorithm which will be the bees knees
   once I get it wired to clang.

llvm-svn: 65377