[Ada] Improve detection of illegal Iterable aspects

[Ada] Improve detection of illegal Iterable aspects

Handling of aspect Iterable was lacking guards against illegal code, so
the compiler either crashed or emitted cryptic errors while expanding
loops that rely on this aspect.

gcc/ada/

	* doc/gnat_rm/implementation_defined_aspects.rst
	(Aspect Iterable): Include Last and Previous primitives in
	syntactic and semantic description.
	* exp_attr.adb
	(Expand_N_Attribute_Reference): Don't expect attributes like
	Iterable that can only appear in attribute definition clauses.
	* sem_ch13.adb
	(Analyze_Attribute_Definition_Clause): Prevent crash on
	non-aggregate Iterable attribute; improve basic diagnosis of
	attribute values.
	(Resolve_Iterable_Operation): Improve checks for illegal
	primitives in aspect Iterable, e.g. with wrong number of formal
	parameters.
	(Validate_Iterable_Aspect): Prevent crashes on syntactically
	illegal aspect expression.
	* sem_util.adb
	(Get_Cursor_Type): Fix style.
	* gnat_ugn.texi, gnat_rm.texi: Regenerate.