Implement empty base optimization.

Implement empty base optimization.

	* class.c (finish_struct_1): Add vbase fields earlier.  Set
	CLASSTYPE_SIZE of an empty base to 0.  Types with bases can be empty.
	* search.c (dfs_check_overlap, dfs_no_overlap_yet): New fns.
	(types_overlap_p): New fn.
	* tree.c (avoid_overlap): New fn.
	(build_base_fields): Use it to avoid overlapping empty bases.
	* cp-tree.h, decl2.c, lang-options.h: Add -fnew-abi.

From-SVN: r18978