[Ada] Avoid copy operation for returns involving function calls

[Ada] Avoid copy operation for returns involving function calls

The underlying issue is that the front-end does not create transient scopes
for return statements, so objects copied for these statements can never be
finalized properly.

gcc/ada/

	* exp_ch6.adb (Expand_Call_Helper): Adjust comment.
	(Expand_Simple_Function_Return): For the case of a type which needs
	finalization and is returned on the primary stack, do not create a
	copy if the expression originates from a function call.
	* exp_ch7.adb (Transient Scope Management): Adjust comment.
	* exp_util.ads (Is_Related_To_Func_Return): Add WARNING line.
	* fe.h (Is_Related_To_Func_Return): Declare.