[Ada] Move duplicated routines for building itypes to Sem_Util

[Ada] Move duplicated routines for building itypes to Sem_Util

2020-06-11  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* sem_aggr.adb (Build_Constrained_Itype): Move to Sem_Util.
	* sem_ch3.adb (Build_Subtype, Inherit_Predicate_Flags): Move...
	* sem_util.adb (Build_Subtype): Here.  Add parameters for
	references to objects previously declared in enclosing scopes.
	(Inherit_Predicate_Flags): And here, because it is called by
	Build_Subtype.
	* sem_util.ads (Build_Overriding_Spec): Reorder alphabetically.
	(Build_Subtype): Moved from Sem_Ch3; comments updated.
	(Build_Constrained_Itype): Moved from Sem_Aggr; comments
	updated.