Enable clang to continue to parse libstdc++4.6 and stlport after r210091.

r210091 made initialization checking more strict in c++11 mode. LWG2193 is
about changing standard libraries to still be valid under these new rules,
but older libstdc++ (e.g. libstdc++4.6 in -D_GLIBCXX_DEBUG=1 mode, or stlport)
do not implement that yet.  So fall back to the C++03 semantics for container
classes in system headers below the std namespace.