[Ada] Implement AI12-0290 (Simple_Barriers restriction)

[Ada] Implement AI12-0290 (Simple_Barriers restriction)

2020-06-08  Steve Baird  <baird@adacore.com>

gcc/ada/

	* sem_ch13.ads: Export new function
	All_Membership_Choices_Static.
	* sem_ch13.adb: Implement new function
	All_Membership_Choices_Static.  This involves moving the
	functions Is_Static_Choice and All_Membership_Choices_Static,
	which were previously declared within the function
	Is_Predicate_Static, out to library level so that they can be
	called by the new function. The already-exisiting code in
	Is_Predicate_Static which became the body of
	All_Membership_Choices_Static is replaced with a call to the new
	function in order to avoid duplication.
	* exp_ch9.adb (Is_Pure_Barrier): Several changes needed to
	implement rules of AI12-0290 and RM D.7's definition of
	"pure-barrier-eligible". These changes include adding a call to
	the new function Sem_13.All_Membership_Choices_Static, as per
	the "see 4.9" in RM D.7(1.6/5).