[Ada] Fix ownership checking for pointers in SPARK

[Ada] Fix ownership checking for pointers in SPARK

Checking of the readable status of sub-expressions occurring in the
target path of an assignment should occur before the right-hand-side is
moved or borrowed or observed.

There is no impact on compilation.

2019-07-09  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_spark.adb (Check_Expression): Change signature to take an
	Extended_Checking_Mode, for handling read permission checking of
	sub-expressions in an assignment.
	(Check_Parameter_Or_Global): Adapt to new behavior of
	Check_Expression for mode Assign.
	(Check_Safe_Pointers): Do not analyze generic bodies.
	(Check_Assignment): Separate checking of the target of an
	assignment.

From-SVN: r273266