[Ada] Completion of support for AI12-0409 (attribute Preelaborable_Initialization)

[Ada] Completion of support for AI12-0409 (attribute Preelaborable_Initialization)

gcc/ada/

	* sem_ch7.adb (Analyze_Package_Specification): For types marked
	as Must_Have_Preelab_Init, we now check for the presence of a
	Preelaborable_Initialization aspect on the type, and pass the
	aspect's expression (if any) on the call to
	Has_Preelaborable_Initialization (or pass Empty if the type has
	no such aspect or the aspect has no associated expression).
	* sem_util.ads (Has_Preelaborable_Initialization): Change
	Boolean formal parameter Formal_Types_Have_Preelab_Init to
	instead be a formal of type Node_Id (named Preelab_Init_Expr),
	to allow passing an expression that may be a conjunction of
	Preelaborable_Initialization aspects. Revise spec comment
	accordingly (and remove ??? comment).
	* sem_util.adb (Type_Named_In_Preelab_Init_Expression): New
	nested function with a result indicating whether a given type is
	named as the prefix of a Preelaborable_Initialization attribute
	in the expression of a corresponding P_I aspect.
	(Has_Preelaborable_Initialization): For generic formal derived
	and private types, test whether the type is named in the
	expression Preelab_Init_Expr (by calling
	Type_Named_In_Preelab_Init_Expression), and if so, treat the
	formal type as having preelaborable initialization (returning
	True).
	* libgnat/a-cobove.ads (Vector): Replace pragma
	Preelaborable_Initialization with the aspect, specifying its
	value as Element_Type'Preelaborable_Initialization.
	(Cursor): Replace pragma P_I with the aspect (defaulting to
	True).
	* libgnat/a-cbdlli.ads (List): Replace pragma
	Preelaborable_Initialization with the aspect, specifying its
	value as Element_Type'Preelaborable_Initialization.
	(Cursor): Replace pragma P_I with the aspect (defaulting to
	True).
	* libgnat/a-cbhama.ads (Map): Replace pragma
	Preelaborable_Initialization with the aspect, specifying its
	value as (Element_Type'Preelaborable_Initialization and
	Key_Type'Preelaborable_Initialization).
	(Cursor): Replace pragma P_I with the aspect (defaulting to
	True).
	* libgnat/a-cborma.ads (Map): Replace pragma
	Preelaborable_Initialization with the aspect, specifying its
	value as (Element_Type'Preelaborable_Initialization and
	Key_Type'Preelaborable_Initialization).
	(Cursor): Replace pragma P_I with the aspect (defaulting to
	True).
	* libgnat/a-cbhase.ads (Set): Replace pragma
	Preelaborable_Initialization with the aspect, specifying its
	value as Element_Type'Preelaborable_Initialization.
	(Cursor): Replace pragma P_I with the aspect (defaulting to
	True).
	* libgnat/a-cborse.ads (Set): Replace pragma
	Preelaborable_Initialization with the aspect, specifying its
	value as Element_Type'Preelaborable_Initialization.
	(Cursor): Replace pragma P_I with the aspect (defaulting to
	True).
	* libgnat/a-cbmutr.ads (Tree): Replace pragma
	Preelaborable_Initialization with the aspect, specifying its
	value as Element_Type'Preelaborable_Initialization.
	(Cursor): Replace pragma P_I with the aspect (defaulting to
	True).
	* libgnat/a-coboho.ads (Holder): Replace pragma
	Preelaborable_Initialization with the aspect, specifying its
	value as Element_Type'Preelaborable_Initialization.
	(Cursor): Replace pragma P_I with the aspect (defaulting to
	True).