c++: Adjust pushdecl/duplicate_decls API

c++: Adjust pushdecl/duplicate_decls API

The decl pushing APIs and duplicate_decls take an 'is_friend' parm,
when what they actually mean is 'hide this from name lookup'.  That
conflation has gotten more anachronistic as time moved on.  We now
have anticipated builtins, and I plan to have injected extern decls
soon.  So this patch is mainly a renaming excercise.  is_friend ->
hiding.  duplicate_decls gets an additional 'was_hidden' parm.  As
I've already said, hiddenness is a property of the symbol table, not
the decl.  Builtins are now pushed requesting hiding, and pushdecl
asserts that we don't attempt to push a thing that should be hidden
without asking for it to be hidden.

This is the final piece of groundwork to get rid of a bunch of 'this
is hidden' markers on decls and move the hiding management entirely
into name lookup.

	gcc/cp/
	* cp-tree.h (duplicate_decls): Replace 'is_friend' with 'hiding'
	and add 'was_hidden'.
	* name-lookup.h (pushdecl_namespace_level): Replace 'is_friend'
	with 'hiding'.
	(pushdecl): Likewise.
	(pushdecl_top_level): Drop is_friend parm.
	* decl.c (check_no_redeclaration_friend_default_args): Rename parm
	olddelc_hidden_p.
	(duplicate_decls): Replace 'is_friend' with 'hiding'
	and 'was_hidden'.  Do minimal adjustments in body.
	(cxx_builtin_function): Pass 'hiding' to pushdecl.
	* friend.c (do_friend): Pass 'hiding' to pushdecl.
	* name-lookup.c (supplement_binding_1): Drop defaulted arg to
	duplicate_decls.
	(update_binding): Replace 'is_friend' with 'hiding'.  Drop
	defaulted arg to duplicate_decls.
	(do_pushdecl): Replace 'is_friend' with 'hiding'.  Assert no
	surprise hidhing.  Adjust duplicate_decls calls to inform of old
	decl's hiddennes.
	(pushdecl): Replace 'is_friend' with 'hiding'.
	(set_identifier_type_value_with_scope): Adjust update_binding
	call.
	(do_pushdecl_with_scope): Replace 'is_friend' with 'hiding'.
	(pushdecl_outermost_localscope): Drop default arg to
	do_pushdecl_with_scope.
	(pushdecl_namespace_level): Replace 'is_friend' with 'hiding'.
	(pushdecl_top_level): Drop is_friend parm.
	* pt.c (register_specialization): Comment duplicate_decls call
	args.
	(push_template_decl): Commont pushdecl_namespace_level.
	(tsubst_friend_function, tsubst_friend_class): Likewise.