c++: Kill DECL_HIDDEN_FRIEND_P

c++: Kill DECL_HIDDEN_FRIEND_P

Now hiddenness is managed by name-lookup, we no longer need DECL_HIDDEN_FRIEND_P.
This removes it.  Mainly by deleting its bookkeeping, but there are a couple of uses

1) two name lookups look at it to see if they found a hidden thing.
In one we have the OVERLOAD, so can record OVL_HIDDEN_P.  In the other
we're repeating a lookup that failed, but asking for hidden things --
so if that succeeds we know the thing was hidden.  (FWIW CWG recently
discussed whether template specializations and instantiations should
see such hidden templates anyway, there is compiler divergence.)

2) We had a confusing setting of KOENIG_P when building a
non-dependent call.  We don't repeat that lookup at instantiation time
anyway.

	gcc/cp/
	* cp-tree.h (struct lang_decl_fn): Remove hidden_friend_p.
	(DECL_HIDDEN_FRIEND_P): Delete.
	* call.c (add_function_candidate): Drop assert about anticipated
	decl.
	(build_new_op_1): Drop koenig lookup flagging for hidden friend.
	* decl.c (duplicate_decls): Drop HIDDEN_FRIEND_P updating.
	* name-lookup.c (do_pushdecl): Likewise.
	(set_decl_namespace): Discover hiddenness from OVL_HIDDEN_P.
	* pt.c (check_explicit_specialization): Record found_hidden
	explicitly.