[Ada] Issue advice for error regarding Old/Loop_Entry on unevaluated expr.

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

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Ada puts some restrictions on the use of attributes Old, that SPARK
mimics for attribute Loop_Entry, inside a "potentially unevaluated
context". These restrictions can be lifted by using pragma
Unevaluated_Use_Of_Old with argument Allow. Issue a message to indicate
that to the user.

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

gcc/ada/

	* sem_attr.adb (Analyze_Attribute): Issue a continuation message
	to give proper recommendation here.

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diff --git a/gcc/ada/sem_attr.adb b/gcc/ada/sem_attr.adb
--- a/gcc/ada/sem_attr.adb
+++ b/gcc/ada/sem_attr.adb
@@ -420,9 +420,11 @@ package body Sem_Attr is
       --  no arguments is used when the caller has already generated the
       --  required error messages.
 
-      procedure Error_Attr_P (Msg : String);
+      procedure Error_Attr_P (Msg : String; Msg_Cont : String := "");
       pragma No_Return (Error_Attr_P);
-      --  Like Error_Attr, but error is posted at the start of the prefix
+      --  Like Error_Attr, but error is posted at the start of the prefix. The
+      --  second message Msg_Cont is useful to issue a continuation message
+      --  before raising Bad_Attribute.
 
       procedure Legal_Formal_Attribute;
       --  Common processing for attributes Definite and Has_Discriminants.
@@ -2690,10 +2692,13 @@ package body Sem_Attr is
       -- Error_Attr_P --
       ------------------
 
-      procedure Error_Attr_P (Msg : String) is
+      procedure Error_Attr_P (Msg : String; Msg_Cont : String := "") is
       begin
          Error_Msg_Name_1 := Aname;
          Error_Msg_F (Msg, P);
+         if Msg_Cont /= "" then
+            Error_Msg_F (Msg_Cont, P);
+         end if;
          Error_Attr;
       end Error_Attr_P;
 
@@ -2842,7 +2847,10 @@ package body Sem_Attr is
                        and then Attr_Id = Attribute_Old
                      then " or be eligible for conditional evaluation"
                           & " (RM 6.1.1 (27))"
-                     else ""));
+                     else ""),
+                  Msg_Cont =>
+                    "\using pragma Unevaluated_Use_Of_Old (Allow) will make "
+                    & "this legal");
 
             when 'W' =>
                Error_Msg_Name_1 := Aname;