Lookup and invoke debug methods of C++ classes if they are the best match.

Lookup and invoke debug methods of C++ classes if they are the best match.

	* eval.c (evaluate_subexp_standard): Call the xmethod if the
	best match method returned by find_overload_match is an xmethod.
	* valarith.c (value_x_binop, value_x_unop): Call the xmethod if
	the best matching operator returned by find_overload_match is an
	xmethod.
	* valops.c: #include "extension.h".
	(find_method_list): Add "fn_list" and "xm_worker_vec" arguments.
	Return void.  The list of matching source methods is returned in
	"fn_list" and a vector of matching debug method workers is
	returned in "xm_worker_vec".  Update all callers.
	(value_find_oload_method_list): Likewise.
	(find_oload_champ): Add "xm_worker_vec" parameter.  If it is
	non-NULL, then the index of the best matching method in this
	vector is returned.  Update all callers.
	(find_overload_match): Include xmethods while performing overload
	resolution.