[Ada] Move registering code for predefined primitives to Exp_Disp

[Ada] Move registering code for predefined primitives to Exp_Disp

This avoids making Expand_Interface_Thunk visible from the outside.

No functional changes.

gcc/ada/

	* exp_ch6.adb (Freeze_Subprogram.Register_Predefined_DT_Entry): Move
	procedure to...
	* exp_disp.ads (Expand_Interface_Thunk): Move declaration to...
	(Register_Predefined_Primitive): Declare.
	* exp_disp.adb (Expand_Interface_Thunk): ...here.
	(Register_Predefined_Primitive): ...here and change into a function
	returning List_Id.