[Ada] Further improve the expansion of array aggregates

[Ada] Further improve the expansion of array aggregates

gcc/ada/

	* exp_aggr.adb
	(Convert_To_Positional): Add Dims local variable
	and pass it in calls to Is_Flat and Flatten.
	(Check_Static_Components): Pass Dims in call to
	Is_Static_Element.
	(Nonflattenable_Next_Aggr): New predicate.
	(Flatten): Add Dims parameter and Expr local variable.  Call
	Nonflattenable_Next_Aggr in a couple of places.  In the case
	when an Others choice is present, check that the element is
	either static or a nested aggregate that can be flattened,
	before disregarding the replication limit for elaboration
	purposes.  Check that a nested array is flattenable in the case
	of a multidimensional array in any position.  Remove redundant
	check in the Others case and pass Dims in call to
	Is_Static_Element.  Use Expr variable.
	(Is_Flat): Change type of Dims parameter from Int to Nat.
	(Is_Static_Element): Add Dims parameter.  Replace tests on
	literals with call to Compile_Time_Known_Value.  If everything
	else failed and the dimension is 1, preanalyze the expression
	before calling again Compile_Time_Known_Value on it.  Return
	true for null.
	(Late_Expansion): Do not expand further if the assignment to the
	target can be done directly by the back end.