ada: Give consistent error message for invalid prefix in select component

ada: Give consistent error message for invalid prefix in select component

This gives a consistent error message for an invalid prefix in all the cases
where the prefix has been found to be the name of an entity.

gcc/ada/ChangeLog:

	* sem_ch4.adb (Analyze_Selected_Component): Give a specific error
	message for an invalid prefix that is the dereference of a name.
	* sem_ch8.adb (Find_Selected_Component): Give a consistent error
	message for the name of an invalid prefix.