LLDB can crash if given DWARF debug info for a class that has a base class which isn't a complete definition.

<rdar://problem/15594781>

We need to not crash at any cost. We currently detect if any base classes are forward declarations, emit an error string that directs the use to file a compiler bug, and continues by completing the class with no contents. This avoids a clang crash that would usually follow when we call setBase().