Fix C++20 structural type vs. private base.

Fix C++20 structural type vs. private base.

In my patch to implement C++20 "structural type" I tried to set the access
flags on the artificial base fields appropriately, but failed.  I was
copying TREE_PRIVATE from the binfo, but TREE_PRIVATE on binfo is just a
temporary cache for dfs_access_in_type; we really need to get the
inheritance access information from BINFO_BASE_ACCESSES.

	* class.c (build_base_field_1): Take access parameter.
	(build_base_field): Likewise.
	(build_base_fields, layout_virtual_bases): Pass it.
	* tree.c (structural_type_p): Improve private base diagnostic.

From-SVN: r279184