[Ada] Refactoring related to Returns_By_Ref

[Ada] Refactoring related to Returns_By_Ref

gcc/ada/

	* sem_util.ads, sem_util.adb (Compute_Returns_By_Ref): New
	procedure to compute Returns_By_Ref, to avoid some code
	duplication. This will likely change soon, so it's good to have
	the code in one place.
	(CW_Or_Has_Controlled_Part): Move here from Exp_Ch7, because
	it's called by Compute_Returns_By_Ref, and this is a better
	place for it anyway.
	(Needs_Finalization): Fix comment to be vague instead of wrong.
	* exp_ch6.adb (Expand_N_Subprogram_Body, Freeze_Subprogram):
	Call Compute_Returns_By_Ref.
	* sem_ch6.adb (Check_Delayed_Subprogram): Call
	Compute_Returns_By_Ref.
	* exp_ch7.ads, exp_ch7.adb (CW_Or_Has_Controlled_Part): Move to
	Sem_Util.
	(Has_New_Controlled_Component): Remove unused function.