Simplify ownership by using forward_list<T> rather than vector<unique_ptr<T>>

Since the elements were not polymorphic, the unique_ptr was only used to
avoid pointer invalidation on container resizes - might as well skip the
indirection and use a container with suitable invalidation semantics.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222931 91177308-0d34-0410-b5e6-96231b3b80d8