Instantiate default arguments/member initializers once.

	* init.c (get_nsdmi): Remember NSDMI instantiations.
	* parser.c (inject_this_parameter): Be more picky about
	current_class_ptr.
	* pt.c (tsubst_copy): Simplify 'this' handling.
	(tsubst_default_argument): Remember default argument
	instantiations.  Take parameter number.
	(tsubst_default_arguments): Pass it.
	* call.c (convert_default_arg): Likewise.

From-SVN: r251422