[Ada] Never make symbols for thunks public

[Pierre-Marie de Rodat via Gcc-patches]

Thunks are only referenced locally by dispatch tables and never inlined.

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

gcc/ada/

        * sem_ch6.adb (Analyze_Subprogram_Body_Helper): Clear the Is_Public
        flag on thunks.
diff --git a/gcc/ada/sem_ch6.adb b/gcc/ada/sem_ch6.adb
--- a/gcc/ada/sem_ch6.adb
+++ b/gcc/ada/sem_ch6.adb
@@ -4772,6 +4772,13 @@ package body Sem_Ch6 is
          if Nkind (N) /= N_Subprogram_Body
            or else not Was_Expression_Function (N)
          then
+            --  First clear the Is_Public flag on thunks since they are only
+            --  referenced locally by dispatch tables and thus never inlined.
+
+            if Is_Thunk (Body_Id) then
+               Set_Is_Public (Body_Id, False);
+            end if;
+
             Freeze_Before (N, Body_Id);
          end if;