Improve memory ownership/management in TableGen by unique_ptrifying TreePattern's Tree member.

The next step is to actually use unique_ptr in TreePatternNode's
Children vector. That will be more intrusive, and may not work,
depending on exactly how these things are handled (I have a bad
suspicion things are shared more than they should be, making this more
DAG than tree - but if it's really a tree, unique_ptr should suffice)

llvm-svn: 222183