Add a very primitive clang based multifile 'delta'.

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@65377 91177308-0d34-0410-b5e6-96231b3b80d8