ada: Reject illegal uses of type/subtype current instance

ada: Reject illegal uses of type/subtype current instance

The current instance of a type or subtype (see RM 8.6) is an object or
value, not a type or subtype. So a name denoting such a current instance is
illegal in any context that requires a name denoting a type or subtype.
In some cases this error was not detected.

gcc/ada/

	* sem_ch8.adb (Find_Type): If Is_Current_Instance returns True for
	N (and Comes_From_Source (N) is also True) then flag an error.
	Call Is_Current_Instance (twice) instead of duplicating (twice)
	N_Access_Definition-related code in Is_Current_Instance.
	* sem_util.adb (Is_Current_Instance): Implement
	access-type-related clauses of the RM 8.6 current instance rule.
	For pragmas Predicate and Predicate_Failure, distinguish between
	the first and subsequent pragma arguments.