Core DR 342
	PR c++/48582
	* pt.c (check_valid_ptrmem_cst_expr): A null member pointer value
	is valid in C++11.
	(convert_nontype_argument): Likewise.  Implicitly convert nullptr
	and do constant folding.
	* mangle.c (write_template_arg_literal): Mangle null member
	pointer values as 0.
	* call.c (null_member_pointer_value_p): New.
	* cp-tree.h: Declare it.

From-SVN: r178144