ada: Accessibility code reorganization and bug fixes

ada: Accessibility code reorganization and bug fixes

This patch performs a large reorganization of accessibility related sources,
and also corrects some latent issues with accessibility checks - namely the
calculation of accessibility levels for expanded iterators and type
conversions.

gcc/ada/
	* accessibility.adb, accessibility.ads
	(Accessibility_Message): Moved from sem_attr.
	(Apply_Accessibility_Check): Moved from checks.
	(Apply_Accessibility_Check_For_Allocator): Moved from exp_ch4 and
	renamed
	(Check_Return_Construct_Accessibility): Moved from sem_ch6.
	(Innermost_Master_Scope_Depth): Moved from sem_util. Add condition
	to detect expanded iterators.
	(Prefix_With_Safe_Accessibility_Level): Moved from sem_attr.
	(Static_Accessibility_Level): Moved from sem_util.
	(Has_Unconstrained_Access_Discriminants): Likewise.
	(Has_Anonymous_Access_Discriminant): Likewise.
	(Is_Anonymous_Access_Actual): Likewise.
	(Is_Special_Aliased_Formal_Access): Likewise.
	(Needs_Result_Accessibility_Level): Likewise.
	(Subprogram_Access_Level): Likewise.
	(Type_Access_Level): Likewise.
	(Deepest_Type_Access_Level): Likewise.
	(Effective_Extra_Accessibility): Likewise.
	(Get_Dynamic_Accessibility): Likewise.
	(Has_Access_Values): Likewise.
	(Accessibility_Level): Likewise.
	* exp_attr.adb (Access_Cases): Obtain the proper enclosing object
	which applies to a given 'Access by looking through type
	conversions.
	* exp_ch4.adb (Apply_Accessibility_Check): Moved to accessibility.
	* exp_ch5.adb: Likewise.
	* exp_ch6.adb: Likewise.
	* exp_ch9.adb: Likewise.
	* exp_disp.adb: Likewise.
	* gen_il-fields.ads: Add new flag Comes_From_Iterator.
	* gen_il-gen-gen_nodes.adb: Add new flag Comes_From_Iterator for
	N_Object_Renaming_Declaration.
	* sem_ch5.adb (Analyze_Iterator_Specification): Mark object
	renamings resulting from iterator expansion with the new flag
	Comes_From_Iterator.
	* sem_aggr.adb (Resolve_Container_Aggregate): Refine test.
	* sem_ch13.adb: Add dependence on the accessibility package.
	* sem_ch3.adb: Likewise.
	* sem_ch4.adb: Likewise.
	* sem_ch9.adb: Likewise.
	* sem_res.adb: Likewise.
	* sem_warn.adb: Likewise.
	* exp_ch3.adb: Likewise.
	* sem_attr.adb (Accessibility_Message): Moved to accessibility.
	(Prefix_With_Safe_Accessibility_Level): Likewise.
	* checks.adb, checks.ads (Apply_Accessibility_Check): Likewise.
	* sem_ch6.adb (Check_Return_Construct_Accessibility): Likewise.
	* sem_util.adb, sem_util.ads
	(Accessibility_Level): Likewise.
	(Deepest_Type_Access_Level): Likewise.
	(Effective_Extra_Accessibility): Likewise.
	(Get_Dynamic_Accessibility): Likewise.
	(Has_Access_Values): Likewise.
	(Has_Anonymous_Access_Discriminant): Likewise.
	(Static_Accessibility_Level): Likewise.
	(Has_Unconstrained_Access_Discriminants): Likewise.
	(Is_Anonymous_Access_Actual): Likewise.
	(Is_Special_Aliased_Formal_Access): Likewise.
	(Needs_Result_Accessibility_Level): Likewise.
	(Subprogram_Access_Level): Likewise.
	(Type_Access_Level): Likewise.
	* sinfo.ads: Document new flag Comes_From_Iterator.
	* gcc-interface/Make-lang.in: Add entry for new Accessibility package.