c++: Hiddenness is a property of the symbol table

c++: Hiddenness is a property of the symbol table

This patch moves the handling of decl-hiddenness entirely into the
name lookup machinery, where it belongs.  We need a few new flags,
because pressing the existing OVL_HIDDEN_P into play for non-function
decls doesn't work well.  For a local binding we only need one marker,
as there cannot be both a hidden implicit typedef and a hidden
function.  That's not true for namespace-scope, where they could both
be hidden.

The name-lookup machinery maintains the existing decl_hidden and co
flags, and asserts have been sprinkled around to make sure they are
consistent.  The next series of patches will remove those old markers.
(we'll need to keep one, as there are some special restrictions on
redeclaring friend functions with in-class definitions or default args.)

	gcc/cp/
	* cp-tree.h (ovl_insert): Change final parm to hidden-or-using
	indicator.
	* name-lookup.h (HIDDEN_TYPE_BINDING_P): New.
	(struct cxx_binding): Add type_is_hidden flag.
	* tree.c (ovl_insert): Change using_p parm to using_or_hidden,
	adjust.
	(ovl_skip_hidden): Assert we never see a naked hidden decl.
	* decl.c (xref_tag_1): Delete unhiding friend from here (moved to
	lookup_elaborated_type_1).
	* name-lookup.c (STAT_TYPE_HIDDEN_P, STAT_DECL_HIDDEN_P): New.
	(name_lookup::search_namespace_only): Check new hidden markers.
	(cxx_binding_make): Clear HIDDEN_TYPE_BINDING_P.
	(update_binding): Update new hidden markers.
	(lookup_name_1): Check HIDDEN_TYPE_BINDING_P and simplify friend
	ignoring.
	(lookup_elaborated_type_1): Use new hidden markers.  Reveal the
	decl here.