ada: Handle attributes related to Ada 2012 iterators as internal

Use existing machinery for internal attributes to handle attributes
related to Ada 2012 iterators. All these attributes exist exclusively
as a mean to delay processing.

Code cleanup. The only change in behavior is the wording of error
emitted when one of the internal attributes appears in source code:
from "illegal attribute" (which used to be emitted in the analysis)
to "unrecognized attribute (which is emitted by the parser).

gcc/ada/ChangeLog:

	* exp_attr.adb (Expand_N_Attribute_Reference): Remove explicit
	handling of attributes related to Ada 2012 iterators.
	* sem_attr.adb (Analyze_Attribute, Eval_Attribute): Likewise;
	move attribute Reduce according to alphabetic order.
	* snames.adb-tmpl (Get_Attribute_Id): Add support for new internal
	attributes.
	* snames.ads-tmpl: Recognize names of new internal attributes.
	(Attribute_Id): Recognize new internal attributes.
	(Internal_Attribute_Id): Likewise.
	(Is_Internal_Attribute_Name): Avoid duplication in comment.