[Ada] SPARK support for pointers through ownership

[Ada] SPARK support for pointers through ownership

SPARK RM 3.10 is the final version of the pointer ownership rules. Start
changing the implementation accordingly. Anonymous access types are not
fully supported yet.

There is no impact on compilation.

2019-07-01  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_spark.adb: Completely rework the algorithm for ownership
	checking, as the rules in SPARK RM have changed a lot.
	* sem_spark.ads: Update comments.

From-SVN: r272878