ada: Fix premature finalization for nested return within extended one

ada: Fix premature finalization for nested return within extended one

The return object is incorrectly finalized when the nested return is taken,
because the special flag attached to the return object is not updated.

gcc/ada/

	* exp_ch6.adb (Build_Flag_For_Function): New function made up of the
	code building the special flag for return object present...
	(Expand_N_Extended_Return_Statement): ...in there.  Replace the code
	with a call to Build_Flag_For_Function.  Add assertion for the flag.
	(Expand_Non_Function_Return): For a nested return, if the return
	object needs finalization actions, update the special flag.