[Ada] Implement component finalization ordering rules for type extensions

[Ada] Implement component finalization ordering rules for type extensions

Finalization of a record object is required to finalize any components
that have an access discriminant constrained by a per-object expression
before other components. This includes the case of a type extension;
"early finalization" components of the parent type are required to be
finalized before non-early-finalization extension components. This is
implemented in the extension type's finalization procedure by placing
the call to the parent type's finalization procedure between the
finalization of the "early finalization" extension components and the
finalization of the other extension components. Previously that call was
executed after finalizing all of the extension conponents.

gcc/ada/

	* exp_ch7.adb (Build_Finalize_Statements): Add Last_POC_Call
	variable to keep track of the last "early finalization" call
	generated for type extension's finalization procedure. If
	non-empty, then this will indicate the point at which to insert
	the call to the parent type's finalization procedure. Modify
	nested function Process_Component_List_For_Finalize to set this
	variable (and avoid setting it during a recursive call).  If
	Last_POC_Call is empty, then insert the parent finalization call
	before, rather than after, the finalization code for the
	extension components.