ada: Clean up and restrict usage of Initialization_Statements

ada: Clean up and restrict usage of Initialization_Statements

This mechanism is the only producer of N_Compound_Statement in the expanded
code and parks the statements generated for the in-place initialization of
objects by an aggregate, so that they can be moved to the freeze point if
there is an address aspect/clause, or even cancelled if the aggregate has
been generated for Initialize_Scalars/Normalize_Scalars before a subsequent
pragma Import for the object is encountered.

The main condition for its triggering is that the object be not yet frozen,
but that's always the case when its declaration is being processed, so the
mechanism is triggered unnecessarily and the change restricts this but, on
the other hand, it also extends its usage to the in-place initialization by
a function call, which was implemented by means of a custom deferral.

There should be no functional changes.

gcc/ada/ChangeLog:

	* einfo.ads (Initialization_Statements): Document usage precisely.
	* exp_aggr.adb (Convert_Aggr_In_Object_Decl): Do not create a
	compound statement in most cases, do it only if necessary.
	* exp_ch3.adb (Expand_N_Object_Declaration): Remove a couple of
	useless statements.
	* exp_ch6.adb (Make_Build_In_Place_Call_In_Object_Declaration):
	Use the Initialization_Statements mechanism if necessary.
	* exp_ch7.adb: Remove clauses for Aspects package.
	(Insert_Actions_In_Scope_Around): Use the support code of Exp_Util
	for the Initialization_Statements mechanism.
	* exp_prag.adb (Undo_Initialization): Remove obsolete code.
	* exp_util.ads (Move_To_Initialization_Statements): New procedure.
	(Needs_Initialization_Statements): New function.
	* exp_util.adb (Move_To_Initialization_Statements): New procedure.
	(Needs_Initialization_Statements): New predicate.