[Ada] Fix inaccurate -gnatR output for derived untagged types

[Pierre-Marie de Rodat]

This fixes a couple of quirks in the output generated by the -gnatR switch
for derived untagged types with discriminants:

  1. in normal mode, it would display both the hidden and the visible
     discriminants, which results in overlapping components,

  2. in JSON mode, it would display only the hidden discriminants.

Both modes are changed to display only the visible discriminants.

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

gcc/ada/

        * repinfo.adb (Compute_Max_Length): Skip hidden discriminants.
        (List_Record_Layout): Likewise.
        (List_Structural_Record_Layout): Use First_Discriminant instead
        of First_Stored_Discriminant and Next_Discriminant instead of
        Next_Stored_Discriminant to walk the list of discriminants.
diff --git a/gcc/ada/repinfo.adb b/gcc/ada/repinfo.adb
--- a/gcc/ada/repinfo.adb
+++ b/gcc/ada/repinfo.adb
@@ -1006,10 +1006,12 @@ package body Repinfo is
          Comp := First_Component_Or_Discriminant (Ent);
          while Present (Comp) loop
 
-            --  Skip discriminant in unchecked union (since it is not there!)
+            --  Skip a completely hidden discriminant or a discriminant in an
+            --  unchecked union (since it is not there).
 
             if Ekind (Comp) = E_Discriminant
-              and then Is_Unchecked_Union (Ent)
+              and then (Is_Completely_Hidden (Comp)
+                         or else Is_Unchecked_Union (Ent))
             then
                goto Continue;
             end if;
@@ -1278,10 +1280,12 @@ package body Repinfo is
          Comp := First_Component_Or_Discriminant (Ent);
          while Present (Comp) loop
 
-            --  Skip discriminant in unchecked union (since it is not there!)
+            --  Skip a completely hidden discriminant or a discriminant in an
+            --  unchecked union (since it is not there).
 
             if Ekind (Comp) = E_Discriminant
-              and then Is_Unchecked_Union (Ent)
+              and then (Is_Completely_Hidden (Comp)
+                         or else Is_Unchecked_Union (Ent))
             then
                goto Continue;
             end if;
@@ -1370,7 +1374,7 @@ package body Repinfo is
             Derived_Disc : Entity_Id;
 
          begin
-            Derived_Disc := First_Stored_Discriminant (Outer_Ent);
+            Derived_Disc := First_Discriminant (Outer_Ent);
 
             --  Loop over the discriminants of the extension
 
@@ -1394,7 +1398,7 @@ package body Repinfo is
                   end if;
                end if;
 
-               Next_Stored_Discriminant (Derived_Disc);
+               Next_Discriminant (Derived_Disc);
             end loop;
 
             --  Disc is not constrained by a discriminant of Outer_Ent
@@ -1463,12 +1467,13 @@ package body Repinfo is
                end if;
 
                --  If the record has discriminants and is not an unchecked
-               --  union, then display them now.
+               --  union, then display them now. Note that, even if this is
+               --  a structural layout, we list the visible discriminants.
 
                if Has_Discriminants (Ent)
                  and then not Is_Unchecked_Union (Ent)
                then
-                  Disc := First_Stored_Discriminant (Ent);
+                  Disc := First_Discriminant (Ent);
                   while Present (Disc) loop
 
                      --  If this is a record extension and the discriminant is
@@ -1506,7 +1511,7 @@ package body Repinfo is
                      List_Component_Layout (Listed_Disc, Indent => Indent);
 
                   <<Continue_Disc>>
-                     Next_Stored_Discriminant (Disc);
+                     Next_Discriminant (Disc);
                   end loop;
                end if;