[Ada] Better error messages for ownership errors in SPARK

[Ada] Better error messages for ownership errors in SPARK

When SPARK code does not follow the ownership rules of SPARK RM 3.10,
the error message now points to a location explaining why the object has
a more restricted permission than the expected one.

There is no impact on compilation.

2019-07-04  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_spark.adb (Explanation, Get_Expl): New functions to get
	the explanation for a permission mismatch.
	(Perm_Error, Perm_Mismatch, Perm_Error_Loop_Exit): Take
	explanation into account for issuing a more precise error
	message.
	(Set_Perm_Prefixes, Set_Perm_Extensions,
	Set_Perm_Extensions_Move): Pass suitable argument for the
	explanation node.

From-SVN: r273050