[lldb] [debugserver] Shut down the exception thread when clearing (llvm#70979)

MachProcess has a MachTask as an ivar. In the MachProcess dtor, we call
MachTask::Clear() to clear its state, before running the dtor of all our
ivars, including the MachTask one.

When we attach on darwin, MachProcess calls
MachTask::StartExceptionThread which does the task_for_pid and then
starts a thread to listen for mach messages. Then MachProcess calls
ptrace(PT_ATTACHEXC). If that ptrace() fails, MachProcess will call
MachTask::Clear. But the exception thread is now up & running and is not
stopped; its ivars will be reset by the Clear() method, and its object
will be freed after the dtor runs.

Actually eliciting a crash in this scenario is very timing sensitive; I
hand-modified debugserver to fail to PT_ATTACHEXC trying to simulate it
on my desktop and was unable. But looking at the source, and an
occasional crash report we've received, it's clear that this is
possible.

rdar://117521198
(cherry picked from commit 66b9283)

Author: jasonmolenda

lldb/tools/debugserver/source/MacOSX/MachTask.mm

@@ -145,6 +145,8 @@
 //----------------------------------------------------------------------
 void MachTask::Clear() {
   // Do any cleanup needed for this task
+  if (m_exception_thread)
+    ShutDownExcecptionThread();
   m_task = TASK_NULL;
   m_exception_thread = 0;
   m_exception_port = MACH_PORT_NULL;