[clang-tidy] Don't leak the TodoCommentHandler object

Preprocessor:addCommentHandler() does not take ownership,
so we'd end up leaking the TodoCommentHandler.

This patch makes it owned by the Check object.

Differential Revision: http://reviews.llvm.org/D5402

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@218068 91177308-0d34-0410-b5e6-96231b3b80d8