[Ada] INOX: prototype alternative accessibility model

[Ada] INOX: prototype alternative accessibility model

gcc/ada/

	* checks.adb (Accessibility_Checks_Suppressed): Add check
	against restriction No_Dynamic_Accessibility_Checks.
	(Apply_Accessibility_Check): Add assertion to check restriction
	No_Dynamic_Accessibility_Checks is not active.
	* debug.adb: Add documentation for new debugging switch to
	control which accessibility model gets employed under
	restriction No_Dynamic_Accessibility_Checks.
	* exp_attr.adb (Expand_N_Attribute_Reference): Disable dynamic
	accessibility check generation when
	No_Dynamic_Accessibility_Checks is active.
	* exp_ch4.adb (Apply_Accessibility_Check): Skip check generation
	when restriction No_Dynamic_Accessibility_Checks is active.
	(Expand_N_Allocator): Disable dynamic accessibility checks when
	No_Dynamic_Accessibility_Checks is active.
	(Expand_N_In): Disable dynamic accessibility checks when
	No_Dynamic_Accessibility_Checks is active.
	(Expand_N_Type_Conversion): Disable dynamic accessibility checks
	when No_Dynamic_Accessibility_Checks is active.
	* exp_ch5.adb (Expand_N_Assignment_Statement): Disable
	alternative accessibility model calculations when computing a
	dynamic level for a SAOAAT.
	* exp_ch6.adb (Add_Call_By_Copy_Code): Disable dynamic
	accessibility check generation when
	No_Dynamic_Accessibility_Checks is active.
	(Expand_Branch): Disable alternative accessibility model
	calculations.
	(Expand_Call_Helper): Disable alternative accessibility model
	calculations.
	* restrict.adb, restrict.ads: Add new restriction
	No_Dynamic_Accessibility_Checks.
	(No_Dynamic_Accessibility_Checks_Enabled): Created to test when
	experimental features (which are generally incompatible with
	standard Ada) can be enabled.
	* sem_attr.adb (Safe_Value_Conversions): Add handling of new
	accessibility model under the restriction
	No_Dynamic_Accessibility_Checks.
	* sem_prag.adb (Process_Restrictions_Or_Restriction_Warnings):
	Disallow new restriction No_Dynamic_Accessibility_Checks from
	being exclusively specified within a body or subunit without
	being present in a specification.
	* sem_res.adb (Check_Fully_Declared_Prefix): Minor comment
	fixup.
	(Valid_Conversion): Omit implicit conversion checks on anonymous
	access types and perform static checking instead when
	No_Dynamic_Accessibility_Checks is active.
	* sem_util.adb, sem_util.ads (Accessibility_Level): Add special
	handling of anonymous access objects, formal parameters,
	anonymous access components, and function return objects.
	(Deepest_Type_Access_Level): When
	No_Dynamic_Accessibility_Checks is active employ an alternative
	model. Add paramter Allow_Alt_Model to override the new behavior
	in certain cases.
	(Type_Access_Level): When No_Dynamic_Accessibility_Checks is
	active employ an alternative model. Add parameter
	Allow_Alt_Model to override the new behavior in certain cases.
	(Typ_Access_Level): Created within Accessibility_Level for
	convenience.
	* libgnat/s-rident.ads, snames.ads-tmpl: Add handing for
	No_Dynamic_Accessibility_Checks.