Guard against the C++ destructor chain by not letting the debugger list clean...

Guard against the C++ destructor chain by not letting the debugger list clean up after itself in the C++ destructor chain. 

If users call "static void lldb::SBDebugger::Terminate()" we will clean up the debugger list, and users can individually destroy debugger instances with "static void lldb::SBDebugger::Destroy(SBDebugger &)". But if we let the C++ destructor chain tear down this list, other threads that might still be running as the main thread exits can now crash if they access the debugger list. We stop this by leaking the debugger list and its mutex.

<rdar://problem/26372169>

llvm-svn: 270869