[Ada] Do not clear Is_True_Constant flag on imported constants

[Pierre-Marie de Rodat]

The flag is first set on them as for any other constants but then
cleared when the import pragma is processed.

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

gcc/ada/

        * sem_prag.adb (Process_Import_Or_Interface): Do not
        artificially record a possible modification for a constant.
diff --git a/gcc/ada/sem_prag.adb b/gcc/ada/sem_prag.adb
--- a/gcc/ada/sem_prag.adb
+++ b/gcc/ada/sem_prag.adb
@@ -9136,7 +9136,10 @@ package body Sem_Prag is
 
             Def_Id := Entity (Def_Id);
             Kill_Size_Check_Code (Def_Id);
-            Note_Possible_Modification (Get_Pragma_Arg (Arg1), Sure => False);
+            if Ekind (Def_Id) /= E_Constant then
+               Note_Possible_Modification
+                 (Get_Pragma_Arg (Arg1), Sure => False);
+            end if;
 
          else
             Process_Convention (C, Def_Id);
@@ -9146,7 +9149,10 @@ package body Sem_Prag is
 
             Mark_Ghost_Pragma (N, Def_Id);
             Kill_Size_Check_Code (Def_Id);
-            Note_Possible_Modification (Get_Pragma_Arg (Arg2), Sure => False);
+            if Ekind (Def_Id) /= E_Constant then
+               Note_Possible_Modification
+                 (Get_Pragma_Arg (Arg2), Sure => False);
+            end if;
          end if;
 
          --  Various error checks