[TableGen] Make getOnlyTree return a const ref (NFC)

This avoids some unnecessary copies of shared_ptrs.
Those changes were suggested post-commit for D47463.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334656 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed