Add --enable-cxx11 to llvm builds.  As of r195239, SmallPtrSet.h /
SmallPtrSet.cpp use different methods if SmallPtrSet.h is included
in C++11 mode.  Building llvm in C++03 mode and lldb in C++11 mode
resulted in a link-time failure with the C++11-mode-specific method
not being found in the llvm build.