[Ada] Reject allocators in contexts restricted by SPARK

[Pierre-Marie de Rodat]

References to allocators, just like references to volatile objects, are
now rejected in all context restricted by SPARK, both within actual
parameters and outside of them.

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

gcc/ada/

        * sem_ch4.adb (Analyze_Allocator): Reject allocators in
        restricted contexts.
diff --git a/gcc/ada/sem_ch4.adb b/gcc/ada/sem_ch4.adb
--- a/gcc/ada/sem_ch4.adb
+++ b/gcc/ada/sem_ch4.adb
@@ -889,6 +889,16 @@ package body Sem_Ch4 is
          Check_Restriction (No_Local_Allocators, N);
       end if;
 
+      if SPARK_Mode = On
+        and then Comes_From_Source (N)
+        and then not Is_OK_Volatile_Context (Context       => Parent (N),
+                                             Obj_Ref       => N,
+                                             Check_Actuals => False)
+      then
+         Error_Msg_N
+           ("allocator cannot appear in this context (SPARK RM 7.1.3(10))", N);
+      end if;
+
       if Serious_Errors_Detected > Sav_Errs then
          Set_Error_Posted (N);
          Set_Etype (N, Any_Type);