ada: Do not create temporaries for initialization statements

Assignment statements marked with the No_Ctrl_Actions or No_Finalize_Actions
flag are initialization statements and, therefore, no temporaries are needed
to hold the value of the right-hand side for them.

gcc/ada/ChangeLog:

	* gcc-interface/trans.cc (Call_to_gnu): Always use the return slot
	optimization if the parent node is an initialization statement.
	(gnat_to_gnu) <N_Assignment_Statement>: Build an INIT_EXPR instead
	of a MODIFY_EXPR if this is an initialization statement.