[clang-diff] Fix similarity computation

Summary:
Add separate tests for the top-down and the bottom-up phase, as well as
one for the optimal matching.

Reviewers: arphaman

Subscribers: klimek

Differential Revision: https://reviews.llvm.org/D36185

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@311284 91177308-0d34-0410-b5e6-96231b3b80d8