ada: Initial implementation of Extended_Access aspect (FE portion only)

The Extended_Access aspect can be specified to be True for certain
access-to-unconstrained-array-subtype types. Such extended access types
can designate objects that a normal general access type (with the same
designated subtype) cannot, such as a slice of an aliased array object
or an object that is represented without contiguous bounds information.

gcc/ada/ChangeLog:

	* aspects.ads: Add Aspect_Extended_Access to Aspect_Id
	enumeration.
	* par-prag.adb: Add Pragma_Extended_Access to list of pragmas that
	get no interesting processing in the parser.
	* sem_attr.adb: Relax legality checks on Access/Unchecked_Access
	attribute references if access type is Extended_Access.
	* sem_ch12.adb (Validate_Access_Type_Instance): For an instance of
	a generic with a formal access type, check that formal and actual
	agree with with respect to Extended_Access aspect.
	* sem_prag.adb (Analyze_Pragma): Add analysis code for pragma
	Extended_Access. Set Pragma_Extended_Access element in Sig_Flags
	aggregate.
	* sem_prag.ads: Set Pragma_Extended_Access element in
	Aspect_Specifying_Pragma aggregate.
	* sem_res.adb (Valid_Conversion): Disallow
	extended-to-not-extended access conversion.
	* sem_util.adb (Is_Extended_Access_Access_Type): Implement new
	function.
	(Is_Aliased_View): If (and only if) the new Boolean For_Extended
	parameter is True, then a slice of an aliased non-bitpacked array
	is aliased, a constrained nominal subtype does not force a result
	of False, and a dereference of an extended access value is
	aliased. The last point is somewhat subtle. This is how we prevent
	covert fat-to-nonfat type conversions via things like
	"Not_Extended_Type'(Extended_Ptr.all'Access)" or passing
	Extended_Ptr.all as an actual parameter corresponding to an
	explicitly aliased formal parameter.
	* sem_util.ads (Is_Extended_Access_Type): Declare new function.
	(Is_Aliased_View): Add new defaults-False parameter For_Extended.
	* snames.ads-tmpl: Declare Name_Extended_Access Name_Id constant
	and Pragma_Extended_Access Pragma_Id enumeration literal.

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