[12.x] Simplify clearing resolved instances for Facade classes

[jnoordsij]

If one currently wants to clear the resolved instance for a Facade class, there are two possibilities:

• Find out (through the implementing code) what the value of getFacadeAccessor for the particular class is, then pass this to Facade::clearResolvedInstance
• Clear all instances with clearResolvedInstances

Given the first is somewhat cumbersome and the second a bit too much of a shotgun approach for some usecases, this PR propose to rework the clearResolvedInstance instance method in such a way that it allows calling it without any arguments, defaulting to the value obtained from getFacadeAccessor. This allows one to simply call ChildFacade::clearResolvedInstance() without requiring any knowledge about the underlying accessor implementation.

For 'stylistic' reasons, I've opted to change relevant calls to:

• Facade::clearResolvedInstances to signify all instances will be cleared, regardless of class
• ChildFacade::clearResolvedInstance to signify which particular instance will be cleared