Use std::unique_ptr to make the ownership explicit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214377 91177308-0d34-0410-b5e6-96231b3b80d8
A std::unique_ptr case I missed in the previous patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214379 91177308-0d34-0410-b5e6-96231b3b80d8