Expand the DebugInterface to include a function for clearing all of the memchecks.

Breakpoints have one, but memchecks don't, despite being cleared directly in the breakpoint window.

Also added the override keyword to the virtual functions in DSPDebugInterface.cpp. Makes it more explicit that we're overriding (if the name wasn't evident enough).

Now DolphinWX should call the interface functions and not the direct functions of the breakpoints or memchecks for clearing.