[Ada] Fix unnesting crash with Predicate_Failure/no pred

[Ada] Fix unnesting crash with Predicate_Failure/no pred

2020-06-11  Bob Duff  <duff@adacore.com>

gcc/ada/

	* sem_ch13.adb (Analyze_Aspect_Specifications): Do not set the
	Has_Predicates flag when the Predicate_Failure aspect is seen.
	It is legal (but pointless) to use this aspect without a
	predicate.  If we set the flag, we generate a half-baked
	Predicate procedure, and if that procedure is nested, it causes
	unnesting to crash.