[Ada] Fix recent regression on array aggregate with dynamic subtype

[Ada] Fix recent regression on array aggregate with dynamic subtype

This prevents either a crash or an assertion failure in gigi on an array
with dynamic subtype that is wrongly flagged as static by the front-end
because of a recent improvement made in the handling of nested
aggregates.

The patch reuses the existing Static_Array_Aggregate predicate instead
of fixing the problematic test, pluging a few loopholes in the process.
The predicate is conservatively correct but should be good enough in
practice.

2018-12-03  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_aggr.adb (Convert_To_Positional): Use
	Static_Array_Aggregate to decide whether to set
	Compile_Time_Known_Aggregate on an already flat aggregate.
	(Expand_Array_Aggregate): Remove test on
	Compile_Time_Known_Aggregate that turns out to be dead and
	simplify.
	(Is_Static_Component): New predicate extracted from...
	(Static_Array_Aggregate): ...here.  Test neither Is_Tagged_Type
	nor Is_Controlled for the type, but test whether the component
	type has discriminants.  Use the Is_Static_Component predicate
	consistently for the positional and named cases.

gcc/testsuite/

	* gnat.dg/array32.adb, gnat.dg/array32.ads: New testcase.

From-SVN: r266755