ada: Fix Initialize aspect for constructors

ada: Fix Initialize aspect for constructors

This patch improve the analysis for the Initialize aspect in constructor bodies.
Specifically:
- Assignments based on the Initialize aspect are always placed at the end of the
  constructor prologue, otherwise they could be overwritten depending on the
  original order of components.
- Introduce the "others" default choice for the Initialize aggregate.
- Improve diagnostics when the Initialize aspect is clearly misspelled.
- Flag components that are required to be initialized but are missing from the
  Initialize aspect.
- Check whether aggregate choices refer to ancestors, which is not allowed.

gcc/ada/ChangeLog:

	* exp_ch3.adb (Build_Implicit_Parameterless_Constructor): Add Initialize
	aspect with default others to trigger Initialize aspect analysis.
	* exp_ch6.adb (Prepend_Constructor_Procedure_Prologue): Fix
	initialization order.
	(Init_From_Initialize_Expression): Retrieve initialization expression or
	the default one base on the Initialize aspect.
	(Init_From_Default_Or_Constructor):. Retrieve initialization expression
	based on the default one in the record initialization list or the init
	procedure.
	* sem_ch13.adb (Analyze_Aspect_Specifications): Add check for
	missing components that require initialization, and add an
	expression_with_action node to place ABE during resolution of
	aggregates with function calls.
	(Check_Constructor_Choices): Helper to check that the aggregate choices
	do not refer to ancestors.
	(Diagnose_Misplaced_Aspects): Improve diagnostics when it is a clear
	misspelling of Initialize aspect.
	* sem_ch6.adb (Analyze_Direct_Attribute_Definition): If missing, add a
	compiler generated Initialize aspect with default others to trigger
	Initialize analysis.