[Ada] Fix crash for Default_Initial_Condition on derived enumeration type

[Ada] Fix crash for Default_Initial_Condition on derived enumeration type

This fixes a crash on the declaration of a private derived enumeration type
with the Default_Initial_Condition aspect and in the process makes a couple
of related adjustments: 1) removes the early freezing of implicit character
and numeric base types and 2) fixes an oversight in the implementation of
delayed representation aspects.

gcc/ada/

	* aspects.ads (Delaying Evaluation of Aspect): Fix typos.
	* exp_ch3.adb (Freeze_Type): Do not generate Invariant and DIC
	procedures for internal types.
	* exp_util.adb (Build_DIC_Procedure_Body): Adjust comment.
	* freeze.adb (Freeze_Entity): Call Inherit_Delayed_Rep_Aspects for
	subtypes and derived types only after the base or parent type has
	been frozen.  Remove useless freezing for first subtype.
	(Freeze_Fixed_Point_Type): Call Inherit_Delayed_Rep_Aspects too.
	* layout.adb (Set_Elem_Alignment): Deal with private types.
	* sem_ch3.adb (Build_Derived_Enumeration_Type): Build the implicit
	base as an itype and do not insert its declaration in the tree.
	(Build_Derived_Numeric_Type): Do not freeze the implicit base.
	(Derived_Standard_Character): Likewise.
	(Constrain_Enumeration): Inherit the chain of representation items
	instead of replacing it.
	* sem_ch13.ads (Inherit_Aspects_At_Freeze_Point): Add ??? comment.
	(Inherit_Delayed_Rep_Aspects): Declare.
	* sem_ch13.adb (Analyze_Aspects_At_Freeze_Point): Do not invoke
	Inherit_Delayed_Rep_Aspects.
	(Inherit_Aspects_At_Freeze_Point): Deal with private types.
	(Inherit_Delayed_Rep_Aspects): Move to library level.