[Ada] Fix a couple of oversights in previous change

[Pierre-Marie de Rodat]

This adds a missing guard in Make_Final_Call for illegal cases and make
sure Build_Derived_Private_Type does a full derivation in the new
handled cases.

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

2020-06-04  Eric Botcazou  <ebotca...@adacore.com>

gcc/ada/

        * exp_ch7.adb (Make_Final_Call): Add missing guard.
        * sem_ch3.adb (Copy_And_Build): Adjust recursive call for
        private types.
        (Build_Derived_Private_Type): Deal with underlying full views.
--- gcc/ada/exp_ch7.adb
+++ gcc/ada/exp_ch7.adb
@@ -8290,6 +8290,7 @@ package body Exp_Ch7 is
          Ref  := Convert_Concurrent (Ref, Typ);
 
       elsif Is_Private_Type (Typ)
+        and then Present (Underlying_Type (Typ))
         and then Is_Concurrent_Type (Underlying_Type (Typ))
       then
          Utyp := Corresponding_Record_Type (Underlying_Type (Typ));

--- gcc/ada/sem_ch3.adb
+++ gcc/ada/sem_ch3.adb
@@ -7735,9 +7735,13 @@ package body Sem_Ch3 is
                end if;
 
             else
+               --  If the parent type is private, this is not a completion and
+               --  we build the full derivation recursively as a completion.
+
                Build_Derived_Type
                  (Full_N, Full_Parent, Full_Der,
-                  Is_Completion => False, Derive_Subps => False);
+                  Is_Completion => Is_Private_Type (Full_Parent),
+                  Derive_Subps => False);
             end if;
 
             --  The full declaration has been introduced into the tree and
@@ -7925,7 +7929,9 @@ package body Sem_Ch3 is
          --  case (see point 5. of its head comment) since we build it for the
          --  derived subtype.
 
-         if Present (Full_View (Parent_Type))
+         if (Present (Full_View (Parent_Type))
+             or else (Present (Underlying_Full_View (Parent_Type))
+                       and then Is_Completion))
            and then not Is_Itype (Derived_Type)
          then
             declare
@@ -7977,8 +7983,14 @@ package body Sem_Ch3 is
             end;
          end if;
 
-      elsif Present (Full_View (Parent_Type))
-        and then Has_Discriminants (Full_View (Parent_Type))
+      elsif (Present (Full_View (Parent_Type))
+              and then
+             Has_Discriminants (Full_View (Parent_Type)))
+        or else (Present (Underlying_Full_View (Parent_Type))
+                  and then
+                 Has_Discriminants (Underlying_Full_View (Parent_Type))
+                  and then
+                 Is_Completion)
       then
          if Has_Unknown_Discriminants (Parent_Type)
            and then Nkind (Subtype_Indication (Type_Definition (N))) =