Refactoring cl::list_storage from "is a" to "has a" std::vector.

Summary: This isn't necessarily an ideal change, and I want to at least reduce the API surface area, but for the new API we really shouldn't be relying on cl::list being a std::vector.

Reviewers: chandlerc

Reviewed By: chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10093

llvm-svn: 238485