
Allow deque and deque::iterator instantiation with incomplete element type. This is an ABI breaking change, and it is only enabled if LIBCXX_ABI_VERSION >= 2 or LIBCXX_ABI_UNSTABLE=ON. git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@252350 91177308-0d34-0410-b5e6-96231b3b80d8