[libcxx] Fix for ALL undefined behavior in <list>.

Summary:
This patch fixes std::list for builtin pointer types in the current ABI version and fixes std::list for all fancy pointer types in the next ABI version. The patch was designed to minimize the amount of code needed to support both ABI configurations. Currently only ~5 lines of code differ.


Reviewers: danalbert, jroelofs, mclow.lists

Subscribers: dexonsmith, awi, cfe-commits

Differential Revision: http://reviews.llvm.org/D12299

llvm-svn: 256652