Revert "Improve memory ownership/management in TableGen by unique_ptrifying...

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.

llvm-svn: 222187