Clean up MachTask.mm's handling of m_exception_thread.

This was getting joined in ShutDownExcecptionThread (sic) but not
cleared.  So this function was not safe to call twice, since you
aren't supposed to join a thread twice.  Sadly, this was called in
MachTask::Clear and MachTask::Destroy, which are both called when
you tell debugserver to detach.

This didn't seem to cause problems IRL, but the most recent ASAN
detects this as an error and calls ASAN::Die, which was causing all
the tests that ran detach to fail.

I fixed that by moving the clear & test for m_exception_thread to
ShutDownExceptionThread.  I also fixed the spelling of that routine.
And that routine was claiming to return a kern_return_t which no one
was checking.  It actually returns a kern_return_t if there was a
Mach failure and a Posix error if there was a join failure.  Since
there's really nothing you can do but exit if this fails, which is
always what you are in the process of doing when you call this, and
since we have already done all the useful logging in
ShutDownExceptionThread, I just removed the return value.

Author: jimingham

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

@@ -1739,7 +1739,7 @@ static uint64_t bits(uint64_t value, uint32_t msbit, uint32_t lsbit) {
     ReplyToAllExceptions();
   }
 
-  m_task.ShutDownExcecptionThread();
+  m_task.ShutDownExceptionThread();
 
   // Detach from our process
   errno = 0;

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

@@ -68,7 +68,7 @@ class MachTask {
   bool ExceptionPortIsValid() const;
   kern_return_t SaveExceptionPortInfo();
   kern_return_t RestoreExceptionPortInfo();
-  kern_return_t ShutDownExcecptionThread();
+  void ShutDownExceptionThread();
 
   bool StartExceptionThread(
       const RNBContext::IgnoredExceptions &ignored_exceptions, DNBError &err);

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

@@ -145,10 +145,8 @@
 //----------------------------------------------------------------------
 void MachTask::Clear() {
   // Do any cleanup needed for this task
-  if (m_exception_thread)
-    ShutDownExcecptionThread();
+  ShutDownExceptionThread();
   m_task = TASK_NULL;
-  m_exception_thread = 0;
   m_exception_port = MACH_PORT_NULL;
   m_exec_will_be_suspended = false;
   m_do_double_resume = false;
@@ -685,8 +683,11 @@ static void get_threads_profile_data(DNBProfileDataScanType scanType,
   return false;
 }
 
-kern_return_t MachTask::ShutDownExcecptionThread() {
+void MachTask::ShutDownExceptionThread() {
   DNBError err;
+  
+  if (!m_exception_thread)
+    return;
 
   err = RestoreExceptionPortInfo();
 
@@ -702,6 +703,8 @@ static void get_threads_profile_data(DNBProfileDataScanType scanType,
   if (DNBLogCheckLogBit(LOG_TASK) || err.Fail())
     err.LogThreaded("::pthread_join ( thread = %p, value_ptr = NULL)",
                     m_exception_thread);
+                    
+  m_exception_thread = nullptr;
 
   // Deallocate our exception port that we used to track our child process
   mach_port_t task_self = mach_task_self();
@@ -713,7 +716,7 @@ static void get_threads_profile_data(DNBProfileDataScanType scanType,
   m_exec_will_be_suspended = false;
   m_do_double_resume = false;
 
-  return err.Status();
+  return;
 }
 
 void *MachTask::ExceptionThread(void *arg) {