<rdar://problem/13443931>

Fixed a crasher in the SourceManager where it wasn't checking the m_target member variable for NULL.

In doing this fix, I hardened this class to have weak pointers to the debugger and target in case they do go away. I also changed SBSourceManager to hold onto weak pointers to the debugger and target so they don't keep objects alive by holding a strong reference to them.

llvm-svn: 177365