[Ada] Fix spurious ownership error in GNATprove

[Ada] Fix spurious ownership error in GNATprove

Like Is_Path_Expression, function Is_Subpath_Expression should consider
the possibility that the subpath is a type conversion or type
qualification over the actual subpath node. This avoids spurious
ownership errors in GNATprove.

There is no impact on compilation.

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

gcc/ada/

	* sem_spark.adb (Is_Subpath_Expression): Take into account
	conversion and qualification.

From-SVN: r274452