[Ada] Implement AI12-0086's rules for discriminants in aggregates

In Ada2012, a discriminant value that governs an active variant part in
an aggregate had to be static. AI12-0086 relaxes this restriction - if
the subtype of the discriminant value is a static subtype all of whose
values select the same variant, then that is good enough.

2019-09-18  Steve Baird  <baird@adacore.com>

gcc/ada/

	* sem_util.ads (Interval_Lists): A new visible package. This
	package is visible because it is also intended for eventual use
	in Sem_Eval.Subtypes_Statically_Compatible when that function is
	someday upgraded to handle static predicates correctly.  This
	new package doesn't really need to be visible for now, but it
	still seems like a good idea.
	* sem_util.adb (Gather_Components): Implement AI12-0086 via the
	following strategy. The existing code knows how to take a static
	discriminant value and identify the corresponding variant; in
	the newly-permitted case of a non-static value of a static
	subtype, we arbitrarily select a value of the subtype and find
	the corresponding variant using the existing code. Subsequently,
	we check that every other value of the discriminant's subtype
	corresponds to the same variant; this is done using the newly
	introduced Interval_Lists package.
	(Interval_Lists): Provide a body for the new package.

gcc/testsuite/

	* gnat.dg/ai12_0086_example.adb: New testcase.

From-SVN: r275857

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