[Ada] Implement late initialization rules for type extensions

[Ada] Implement late initialization rules for type extensions

Default initialization of a record object is required to initialize any
components that "require late initialization" after other components.
This includes the case of a type extension; "late initialization"
components of the parent type are required to be initialized after
non-late-init extension components. This is implemented by generalizing
the use of an existing init proc parameter. Previously, the init proc
for a tagged type took a Boolean parameter indicating whether or not to
initialize the Tag component. With this change, this parameter can now
take on any of four values indicating whether to perform

   1) Full initialization (including the tag component).
   2) Full initialization except for the tag component.
   3) Early (non-tag) initialization only.
   4) Late (non-tag) initialization only.

With this change, the init proc for a type extension has the option of
performing the early and late portions of the parent's initialization
via two separate calls to the parent type's init proc.

gcc/ada/

	* exp_ch3.ads (Build_Intialization_Call): Add new formal
	parameter, Init_Control_Actual, with default value. Clients
	outside of package Exp_Ch3 are unaffected.
	* exp_ch3.adb (Initialization_Control): new package; support for
	this 4-valued parameter.  The existing Requires_Late_Init
	function is moved into this new package.
	(Build_Initialization_Call): Add new formal parameter for
	subprogram body, use this new formal parameter in generating an
	init proc call.
	(Build_Record_Init_Proc): Replace Set_Tag Boolean formal
	parameter with 4-valued Init_Control_Formal. Wrap if-statements
	with appropriate conditions around tag initialization, early
	initialization, and late initialization statements.
	* exp_util.adb (Build_Task_Image_Decl): Avoid problem with
	duplicate declarations when an init proc for a type extension
	calls the parent type's init proc twice.