ada: Improve error message for incorrect call arguments

Enumeration literals are considered as zero argument functions.
This is not obvious for most users and can create confusing error
messages when an enumeration literal is referred to as a call. For
example if there are indices applied to it.

Instead create a dedicated error message for this scenario.

gcc/ada/ChangeLog:

	* sem_util.adb (Normalize_Actuals): Improve error message.