Implement D1907R1 "structural type".

Implement D1907R1 "structural type".

ISO C++ paper D1907R1 proposes "structural type" as an alternative to the
current notion of "strong structural equality", which has various problems.
I'm implementing it to give people a chance to try it.

The build_base_field changes are to make it easier for structural_type_p to
see whether a base is private or protected.

	* tree.c (structural_type_p): New.
	* pt.c (invalid_nontype_parm_type_p): Use it.
	* class.c (build_base_field_1): Take binfo.  Copy TREE_PRIVATE.
	(build_base_field): Pass binfo.

From-SVN: r277902