[Ada] Add second warning for convention C_Variadic_n

[Pierre-Marie de Rodat]

This adds a second warning related to the new C_Variadic_n convention,
for the cases where the aspect/pragma is applied to a subprogram with
exactly n parameters since, in this case, the aspect/pragma is useless.

The warning is given as such:

btest.ads:16:05: warning: subprogram should have at least 4 parameters

for an aspect/pragma C_Variadic_3.

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

2020-06-18  Eric Botcazou  <ebotca...@adacore.com>

gcc/ada/

        * sem_prag.adb (Process_Convention): Give a warning on C_Variadic_n
        being applied to a subprogram with exactly n parameters.
--- gcc/ada/sem_prag.adb
+++ gcc/ada/sem_prag.adb
@@ -8323,11 +8323,21 @@ package body Sem_Prag is
                      Next_Formal (Formal);
                   end loop;
 
+                  --  Error out if the number of parameters is lower than n
+
                   if Count < Minimum then
                      Error_Msg_Uint_1 := UI_From_Int (Minimum);
                      Error_Pragma_Arg
                        ("argument of pragma% must have at least"
                         & "^ parameters", Arg2);
+
+                  --  But warn if it is exactly n because this is useless
+
+                  elsif Count = Minimum then
+                     Error_Msg_Uint_1 := UI_From_Int (Minimum + 1);
+                     Error_Msg_N
+                       ("??subprogram should have at least ^ parameters",
+                        Get_Pragma_Arg (Arg2));
                   end if;
                end;
             end if;