[Ada] Check SPARK restriction on Old/Loop_Entry with pointers

[Ada] Check SPARK restriction on Old/Loop_Entry with pointers

--#! r336866
--#! no-mail

SPARK RM rule 3.10(14) restricts the use of Old and Loop_Entry
attributes on prefixes of an owning or observing type (i.e. a type with
access inside).

There is no impact on compilation.

2019-08-14  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_spark.adb (Check_Old_Loop_Entry): New procedure to check
	correct use of Old  and Loop_Entry.
	(Check_Node): Check subprogram contracts.
	(Check_Pragma): Check Loop_Variant.
	(Check_Safe_Pointers): Apply checking to library-level
	subprogram  declarations as well, in order to check their
	contract.

From-SVN: r274453