[Ada] Set dummy Etype for the fake __HEAP entity in GNATprove

[Ada] Set dummy Etype for the fake __HEAP entity in GNATprove

GNATprove represents reads and writes via pointers as operations on a
fake __HEAP entity. This entity already had various properties set to
dummy values (e.g. Scope set to Standard_Standard), so that it can be
processed like other entities without crashing and not special-cased
everywhere. Now it also has a dummy Etype, so it can be processed with
Is_Single_Concurrent_Object.

The modified code is only executed by GNATprove; frontend is not
affected.

2019-07-08  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* lib-xref-spark_specific.adb (Create_Heap): Set dummy Etype for
	the fake __HEAP entity.

From-SVN: r273216