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

This reverts commit r222183.

Broke on the MSVC buildbots due to MSVC not producing default move
operations - I'd fix it immediately but just broke my build system a
bit, so backing out until I have a chance to get everything going again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222187 91177308-0d34-0410-b5e6-96231b3b80d8