Core 234 - allow const objects with no initializer or user-provided default...

Core 234 - allow const objects with no initializer or user-provided default...

	Core 234 - allow const objects with no initializer or
	user-provided default constructor if the defaulted constructor
	initializes all the subobjects.
	PR c++/20039
	PR c++/42844
	* class.c (default_init_uninitialized_part): New.
	* cp-tree.h: Declare it.
	* decl.c (check_for_uninitialized_const_var): Use it.
	* init.c (perform_member_init): Likewise.
	(build_new_1): Likewise.
	* method.c (walk_field_subobs): Likewise.

From-SVN: r179130