Debugger: Clean up GdbEngine member data on finish

Restarting with cached outdated information is problematic.

Change-Id: I396da2fef96a161c1ab150134b8e65a758a16d58
Task-number: QTCREATORBUG-16355
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/debugger/gdb/gdbengine.h

@@ -72,6 +72,7 @@ private: ////////// General Interface //////////
 
     virtual void handleGdbStartFailed();
     void notifyInferiorSetupFailed() override;
+    void prepareForRestart() override;
 
     bool hasCapability(unsigned) const override;
     void detachDebugger() override;