Revert "Simplify some more ownership using forward_list<T> rather than vector<unique_ptr<T>>"

This reverts commit r222935 and its follow-up r222938 ("Push unique_ptr
a bit further through some APIs and simplify some cleanup"), since it
causes bot failures (at least on Darwin):

http://lab.llvm.org:8080/green/job/clang-stage1-cmake-RA-incremental/1391/