Debugger [CDB]: Release COM interfaces on RunControl finished.

Ensure that at most one instance of the CDBEngine is alive.
Prevent inconsistent debugger states if several sessions are started.
Squashed cherry-picked commits from master correcting the
plugin state handling (2efdeb5cd0,
57f7616286).

Reviewed-by: hjk
Task-number: QTCREATORBUG-2894

src/plugins/debugger/debuggerengine.cpp

@@ -1182,7 +1182,7 @@ void DebuggerEngine::notifyEngineSetupFailed()
     setState(EngineSetupFailed);
     QTC_ASSERT(d->m_runControl, return);
     d->m_runControl->startFailed();
-    d->queueShutdownEngine();
+    setState(DebuggerFinished);
 }
 
 void DebuggerEngine::notifyEngineSetupOk()
@@ -1244,7 +1244,7 @@ void DebuggerEngine::notifyEngineRunFailed()
     d->m_progress.reportCanceled();
     d->m_progress.reportFinished();
     setState(EngineRunFailed);
-    d->queueShutdownInferior();
+    d->queueShutdownEngine();
 }
 
 void DebuggerEngine::notifyEngineRunAndInferiorRunOk()