Streamline implementation of renaming in gigi

Streamline implementation of renaming in gigi

The main changes are 1) the bulk of the implementation is put back entirely
in gnat_to_gnu_entity and 2) the handling of lvalues is unified, i.e. it no
longer depends on the Materialize_Entity flag being present on the entity.

gcc/ada/ChangeLog:
	* gcc-interface/ada-tree.h (DECL_RENAMED_OBJECT): Delete.
	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Variable>: Always use
	the stabilized reference directly for renaming and create a variable
	pointing to it separately if requested.
	* gcc-interface/misc.c (gnat_print_decl): Adjust for deletion.
	* gcc-interface/trans.c (Identifier_to_gnu): Likewise.
	(gnat_to_gnu) <N_Object_Renaming_Declaration>:
	Do not deal with side-effects here.
	<N_Exception_Renaming_Declaration>: Likewise.