[Ada] Issue error on SPARK ownership rule violation

[Ada] Issue error on SPARK ownership rule violation

A modified rule in SPARK RM specifies that object declarations of
anonymous access type should only occur immediately in subprogram, entry
or block. Now checked.

There is no impact on compilation.

2019-07-23  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_spark.ads (Is_Local_Context): New function.
	* sem_spark.adb (Check_Declaration): Issue errors on violations
	of SPARK RM 3.10(4)
	(Process_Path): Do not issue error on borrow/observe during
	elaboration, as these are caught by the new rule.

From-SVN: r273721