Pass a std::unique_ptr& to the create??? methods is lib/Object.

This makes the buffer ownership on error conditions very natural. The buffer
is only moved out of the argument if an object is constructed that now
owns the buffer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211546 91177308-0d34-0410-b5e6-96231b3b80d8