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.

Tested on x86_64-pc-linux-gnu, committed on trunk

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.