[Ada] GNATprove: avoid crash on illegal borrow during package elaboration

[Pierre-Marie de Rodat]

In GNATprove, a borrow during package elaboration is illegal, but a
crash could be issued when analyzing the corresponding declaration. Now
avoid analyzing the declaration in that case.

There is no test as this does not impact compilation.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-09-17  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_spark.adb (Check_Declaration): Do not check the assignment
	from an illegal declaration.
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