decl.c (gnat_to_gnu_entity): Also use return-by-invisible-reference if the return type is By_Reference.

	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Subprogram_Type>: Also
	use return-by-invisible-reference if the return type is By_Reference.
	Tidy up and skip the processing of the return type if it is void.

From-SVN: r166916