[Ada] Implement AI12-0032 fixes for 'Old attribute accessibility

[Pierre-Marie de Rodat <derodat@adacore.com>]

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AI12-0032 modifies/clarifies many of the rules pertaining to uses of the
Old attribute. Most of these 7 changes were already implemented by GNAT
but a couple having to do with accessibility were not. Implement those
unimplemented rules.

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

2020-06-18  Steve Baird  <baird@adacore.com>

gcc/ada/

	* sem_util.adb (Object_Access_Level): Treat a 'Old attribute
	reference like an aggregate in determining its static
	accessibility level; after the evaluation of the relevant
	post-condition, the implicitly declared constant associated with
	an Old attribute reference ceases to exist. Similarly for
	Loop_Entry attribute.
	* exp_ch6.adb (Expand_Call_Helper): For an attribute reference
	that is expanded into a reference to an implicitly declared
	constant (e.g., a 'Old or 'Loop_Entry attribute), compute the
	dynamic accessibility level of that constant by looking at the
	declaration of the constant (as opposed to looking at the
	attribute reference).

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--- gcc/ada/exp_ch6.adb
+++ gcc/ada/exp_ch6.adb
@@ -3617,6 +3617,21 @@ package body Exp_Ch6 is
             then
                Prev_Orig := Prev;
 
+            --  If the actual is an attribute reference that was expanded
+            --  into a reference to an entity, then get accessibility level
+            --  from that entity. AARM 6.1.1(27.d) says "... the implicit
+            --  constant declaration defines the accessibility level of X'Old".
+
+            elsif Nkind (Prev_Orig) = N_Attribute_Reference
+              and then Nam_In (Attribute_Name (Prev_Orig),
+                               Name_Old,
+                               Name_Loop_Entry)
+              and then Is_Entity_Name (Prev)
+              and then Present (Entity (Prev))
+              and then Is_Object (Entity (Prev))
+            then
+               Prev_Orig := Prev;
+
             elsif Nkind (Prev_Orig) = N_Type_Conversion then
                Prev_Orig := Expression (Prev_Orig);
             end if;

--- gcc/ada/sem_util.adb
+++ gcc/ada/sem_util.adb
@@ -24237,6 +24237,18 @@ package body Sem_Util is
       elsif Nkind (Orig_Obj) = N_Aggregate then
          return Object_Access_Level (Current_Scope);
 
+      --  Treat an Old/Loop_Entry attribute reference like an aggregate.
+      --  AARM 6.1.1(27.d) says "... the implicit constant declaration
+      --  defines the accessibility level of X'Old", so that is what
+      --  we are trying to implement here.
+
+      elsif Nkind (Orig_Obj) = N_Attribute_Reference
+        and then Nam_In (Attribute_Name (Orig_Obj),
+                         Name_Old,
+                         Name_Loop_Entry)
+      then
+         return Object_Access_Level (Current_Scope);
+
       --  Otherwise return the scope level of Standard. (If there are cases
       --  that fall through to this point they will be treated as having
       --  global accessibility for now. ???)