[Ada] Attach the special GNATprove HEAP entity to the Standard package

[Pierre-Marie de Rodat]

In GNATprove mode we use frontend cross-references to synthesize the
Global contract of subprograms with SPARK_Mode => Off and represent
a read/write via a pointer as a read/write of a special entity called
HEAP. This entity is now attached to the Standard package, so that we can
safely check the Ekind of its Scope, which now happens in Scope_Within.

This only affects GNATprove, so no frontend test provided.

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

2018-07-17  Piotr Trojanek  <troja...@adacore.com>

gcc/ada/

        * lib-xref-spark_specific.adb (Create_Heap): Attach the HEAP entity to
        the Standard package.
--- gcc/ada/lib-xref-spark_specific.adb
+++ gcc/ada/lib-xref-spark_specific.adb
@@ -287,6 +287,7 @@ package body SPARK_Specific is
 
          Set_Ekind       (Heap, E_Variable);
          Set_Is_Internal (Heap, True);
+         Set_Scope       (Heap, Standard_Standard);
          Set_Has_Fully_Qualified_Name (Heap);
       end Create_Heap;