[Ada] Further Ada 2020 work on accessibility checking

[Pierre-Marie de Rodat]

This patch implements AI12-0035, AI12-0335, AI12-0345, AI12-0372, and
partially Ada2012-A235 and fixes many holes in accessibility checking.

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

gcc/ada/

	* checks.adb (Apply_Accessibility_Check): Skip checks against
	the extra accessibility of a function result when in Ada 2005
	mode or earlier.
	* exp_ch3.adb (Build_Initialization_Call): Modify accessibility
	level calls to use Accessibility_Level.
	(Expand_N_Object_Declaration): Modify accessibility level calls
	to use Accessibility_Level.
	* exp_ch4.adb (Expand_Allocator_Expression): Add static check
	for anonymous access discriminants. Remove unneeded propagation
	of accessibility actual.
	(Expand_N_In): Modify accessibility level calls to use
	Accessibility_Level.
	(Expand_N_Type_Conversion): Modify accessibility level calls to
	use Accessibility_Level.
	* exp_ch5.adb (Expand_N_Assignment_Statement): Modify
	accessibility level calls to use Accessibility_Level.
	* exp_ch6.adb (Expand_Call_Helper): Rewrite accessibility
	calculation for the extra accessibility of result actual in
	function calls, and modify accessibility level calls to use
	Accessibility_Level.
	(Check_Against_Result_Level): Removed.
	* exp_ch9.adb (Expand_N_Requeue_Statement): Add dynamic
	accessibility check for requeues
	* sem_attr.adb (Resolve_Attribute): Modify accessibility level
	calls to use Accessibility_Level.
	* sem_ch13.adb (Associate_Storage_Pool): Modify accessibility
	level calls to use Accessibility_Level.
	* sem_ch4.adb (Analyze_Call): Add static check for explicitly
	aliased formals in function calls within return statements.
	* sem_ch6.adb (Check_Return_Construct_Accessibility): Rewrite
	routine to account for non-aggregate return objects.
	(Generate_Minimum_Accessibility): Created.
	(Analyze_Call): Modify accessibility level calls to use
	Accessibility_Level.
	(Analyze_Subprogram_Body_Helper): Add generation of minimum
	accessibility for the extra accessibility of the function
	result.
	* sem_ch9.adb (Analyze_Requeue): Modify accessibility level
	calls to use Accessibility_Level.
	* sem_res.adb: (Check_Aliased_Parameters): Modify accessibility
	level calls to use Accessibility_Level.
	(Valid_Conversion): Modify accessibility level calls to use
	Accessibility_Level.
	* sem_util.adb, sem_util.ads (Accessibility_Level_Helper):
	Renamed to Accessibility_Level, add detection for functions in
	prefix notation, and add cases where to return zero when
	specified. Modified to take new, more descriptive, parameters.
	(Accessibility_Level): Created.
	(Function_Call_Level): Removed.
	(Function_Call_Or_Allocator_Level): Created to centralize the
	calculation accessibility levels for function calls and
	allocators.
	(Static_Accessibility_Level): Removed.
	(Dynamic_Accessibility_Level): Removed.
	(Get_Dynamic_Accessibility): Renamed from Get_Accessibility.
	(In_Return_Value): Created to determine if a given expression
	contributes to the current function's return value.
	(Is_Master): Created.
	(Is_Explicitly_Aliased): Created
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