[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  <trojanek@adacore.com>

gcc/ada/

	* lib-xref-spark_specific.adb (Create_Heap): Attach the HEAP entity to
	the Standard package.
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