ada: Further cleanup in expansion of array aggregates in allocators

This mainly decouples the handling of the allocator case from that of the
assignment case in Expand_Array_Aggregate and also makes Must_Slide a bit
more forgiving.

gcc/ada/ChangeLog:

	* exp_aggr.adb (In_Place_Assign_OK): Remove handling of allocators
	and call Must_Slide instead of implementing the check manually.
	(Convert_To_Assignments): Adjust outdated comment.
	(Expand_Array_Aggregate): Move handling of allocator case to STEP 3
	and call Must_Slide directly for it.
	(Must_Slide): Replace tests based on Is_OK_Static_Expression with
	tests based on Compile_Time_Known_Value.