Speed up Koenig lookup.

Speed up Koenig lookup.

	* decl.c (unqualified_namespace_lookup): Nonstatic.  Add spacep parm
	to return namespaces we've looked at.
	* decl2.c (lookup_using_namespace): Likewise.
	(add_function): Don't call ovl_member.
	(lookup_arg_dependent): Initialize k.namespaces to the list of
	namespaces seen in unqualified lookup.
	* call.c (equal_functions): Move here from tree.c.
	(joust): Use it to handle duplicate candidates.
	* tree.c (ovl_member): Use ==.

From-SVN: r28710